Bug fix: GLX: Fix --inactive-dim & fix color inversion

- GLX backend: Fix broken --inactive-dim.

- GLX backend: Fix bugs when inverting colors of windows. Thanks to
  madsy and neure for help.

- GLX backend: Lift `glx_no_stencil` restriction from glx_init_blur().
  It still probably won't work, but the user can try.

- XRender backend: Use XRenderFillRectangles() instead of
  XRenderComposite() to do dimming.

src/common.h

@@ -878,9 +878,6 @@ typedef struct _win {
   // Dim-related members
   /// Whether the window is to be dimmed.
   bool dim;
-  /// Picture for dimming. Affected by user-specified inactive dim
-  /// opacity and window opacity.
-  Picture dim_alpha_pict;
 
   /// Whether to invert window color.
   bool invert_color;
@@ -1641,6 +1638,10 @@ bool
 glx_blur_dst(session_t *ps, int dx, int dy, int width, int height, float z,
     GLfloat factor_center);
 
+bool
+glx_dim_dst(session_t *ps, int dx, int dy, int width, int height, float z,
+    GLfloat factor);
+
 bool
 glx_render(session_t *ps, const glx_texture_t *ptex,
     int x, int y, int dx, int dy, int width, int height, int z,