Improvement: GLX: Use SCISSOR_TEST instead of STENCIL_TEST when possible

- GLX backend: Use GL_SCISSOR_TEST instead of STENCIL_TEST if there's
  only one rectangle in glx_set_clip(). Profiling with gDebugger shows a
  10% performance improvement.

- Add .desktop installation rules. (#97)

src/compton.c

@@ -4136,7 +4136,7 @@ usage(void) {
     "--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"
-    "  negative effect on performance.\n"
+    "  negative effect on performance. (My test shows a 10% slowdown.)\n"
 #undef WARNING
 #ifndef CONFIG_DBUS
 #define WARNING WARNING_DISABLED