Improvement: --glx-copy-from-front & benchmark mode

- GLX backend: Add --glx-copy-front, which copies all unmodified regions
  from front buffer to back buffer instead of redrawing the whole
  screen. Unfortunately, probably because glCopyPixels() isn't usually
  well-optimized on graphic cards, this option appears very unstable in
  its effect: Over 20% boost in performance when only 1/4 of the screen
  is modified, but 10% decrease when the whole screen is. Thus, not
  enabled by default.

- GLX backend: Add glx_blur_dst(), to prepare for the background blur
  feature. It currently is capable to modify background in the desired
  way, but the core blur shader is absent. glConvolution2D() seemingly
  relies entirely on CPU and has horrifying performance. I've hesitating
  about whether I should use ARB assembly language or GLSL for the
  shader.

- GLX backend: Invert y-axis GL matrix. It's upside-down previously
  because that's what dcompmgr uses. Seemingly a "normal" y-axis is
  required for glCopyPixels() to operate correctly.

- GLX backend: Fix some GLX_TEXTURE_RECTANGLE compatibility issues.
  Still, not actually tested.

- Add benchmark mode (--benchmark & --benchmark-wid).

- Misc changes.

src/compton.c

@@ -1569,12 +1569,8 @@ paint_all(session_t *ps, XserverRegion region, win *t) {
   XserverRegion reg_paint = None, reg_tmp = None, reg_tmp2 = None;
 
 #ifdef CONFIG_VSYNC_OPENGL
-  // GLX backend: OpenGL doesn't support partial repaint without
-  // GLX_MESA_copy_sub_buffer
   if (BKEND_GLX == ps->o.backend) {
-    free_region(ps, &region);
-    // glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
-    ps->glx_z = 0.0;
+    glx_paint_pre(ps, &region);
   }
 #endif
 
@@ -4134,9 +4130,14 @@ usage(void) {
     "--backend backend\n"
     "  Choose backend. Possible choices are xrender and glx" WARNING ".\n"
     "--glx-no-stencil\n"
-    "  Avoid using stencil buffer under GLX backend. Might cause issues\n"
-    "  when rendering transparent content, may have a positive or\n"
+    "  GLX backend: Avoid using stencil buffer. Might cause issues\n"
+    "  when rendering transparent content. May have a positive or\n"
     "  negative effect on performance. (My test shows a 10% slowdown.)\n"
+    "--glx-copy-from-front\n"
+    "  GLX backend: Copy unmodified regions from front buffer instead of\n"
+    "  redrawing them all. My tests show a 10% decrease in performance\n"
+    "  when the whole screen is modified, but a 20% increase when only 1/4\n"
+    "  is, so this optimization is not enabled by default.\n"
 #undef WARNING
 #ifndef CONFIG_DBUS
 #define WARNING WARNING_DISABLED
@@ -4145,7 +4146,13 @@ usage(void) {
 #endif
     "--dbus\n"
     "  Enable remote control via D-Bus. See the D-BUS API section in the\n"
-    "  man page for more details." WARNING "\n";
+    "  man page for more details." WARNING "\n"
+    "--benchmark cycles\n"
+    "  Benchmark mode. Repeatedly paint until reaching the specified cycles.\n"
+    "--benchmark-wid window-id\n"
+    "  Specify window ID to repaint in benchmark mode. If omitted or is 0,\n"
+    "  the whole screen is repainted.\n"
+    ;
   fputs(usage_text , stderr);
 #undef WARNING
 #undef WARNING_DISABLED
@@ -4594,6 +4601,9 @@ get_cfg(session_t *ps, int argc, char *const *argv, bool first_pass) {
     { "opengl", no_argument, NULL, 289 },
     { "backend", required_argument, NULL, 290 },
     { "glx-no-stencil", no_argument, NULL, 291 },
+    { "glx-copy-from-front", no_argument, NULL, 292 },
+    { "benchmark", required_argument, NULL, 293 },
+    { "benchmark-wid", required_argument, NULL, 294 },
     // Must terminate with a NULL entry
     { NULL, 0, NULL, 0 },
   };
@@ -4860,6 +4870,18 @@ get_cfg(session_t *ps, int argc, char *const *argv, bool first_pass) {
         // --glx-no-stencil
         ps->o.glx_no_stencil = true;
         break;
+      case 292:
+        // --glx-copy-from-front
+        ps->o.glx_copy_from_front = true;
+        break;
+      case 293:
+        // --benchmark
+        ps->o.benchmark = atoi(optarg);
+        break;
+      case 294:
+        // --benchmark-wid
+        ps->o.benchmark_wid = strtol(optarg, NULL, 0);
+        break;
       default:
         usage();
         break;
@@ -5575,7 +5597,7 @@ mainloop(session_t *ps) {
   struct timeval *ptv = NULL;
   {
     // Consider ev_received firstly
-    if (ps->ev_received) {
+    if (ps->ev_received || ps->o.benchmark) {
       ptv = malloc(sizeof(struct timeval));
       ptv->tv_sec = 0L;
       ptv->tv_usec = 0L;
@@ -5658,6 +5680,7 @@ session_init(session_t *ps_old, int argc, char **argv) {
       .display = NULL,
       .backend = BKEND_XRENDER,
       .glx_no_stencil = false,
+      .glx_copy_from_front = false,
       .mark_wmwin_focused = false,
       .mark_ovredir_focused = false,
       .fork_after_register = false,
@@ -5666,6 +5689,8 @@ session_init(session_t *ps_old, int argc, char **argv) {
       .paint_on_overlay = false,
       .unredir_if_possible = false,
       .dbus = false,
+      .benchmark = 0,
+      .benchmark_wid = None,
       .logpath = NULL,
 
       .refresh_rate = 0,
@@ -6249,6 +6274,21 @@ session_run(session_t *ps) {
     while (mainloop(ps))
       continue;
 
+    if (ps->o.benchmark) {
+      if (ps->o.benchmark_wid) {
+        win *w = find_win(ps, ps->o.benchmark_wid);
+        if (!w) {
+          printf_errf("(): Couldn't find specified benchmark window.");
+          session_destroy(ps);
+          exit(1);
+        }
+        add_damage_win(ps, w);
+      }
+      else {
+        force_repaint(ps);
+      }
+    }
+
     // idling will be turned off during paint_preprocess() if needed
     ps->idling = true;
 
@@ -6263,6 +6303,8 @@ session_run(session_t *ps) {
       paint_all(ps, ps->all_damage, t);
       ps->reg_ignore_expire = false;
       paint++;
+      if (ps->o.benchmark && paint >= ps->o.benchmark)
+        exit(0);
       XSync(ps->dpy, False);
       ps->all_damage = None;
     }