Adds ColorMask support to Cogl

This adds CoglPipeline and CoglFramebuffer support for setting a color
mask which is a bit mask defining which color channels should be written
to the current framebuffer.

The final color mask is the intersection of the framebuffer color mask
and the pipeline color mask. The framebuffer mask affects all rendering
to the framebuffer while the pipeline masks can be used to affect
individual primitives.

Reviewed-by: Neil Roberts <neil@linux.intel.com>

cogl/cogl-framebuffer-private.h

@@ -75,6 +75,7 @@ struct _CoglFramebuffer
   int                 alpha_bits;
 
   gboolean            dither_enabled;
+  CoglColorMask       color_mask;
 
   /* We journal the textured rectangles we want to submit to OpenGL so
    * we have an oppertunity to batch them together into less draw