framebuffer: dec current_pipeline_age to force color_mask

When we need to guarantee that the glColorMask is re-asserted the next
time that a primitive is drawn it is not enough to just OR in the
LOGIC_OPS flag to ctx->current_pipeline_changes_since_flush because
_cogl_pipeline_flush_gl_state actually checks the age of the pipeline
before checking that. If the pipeline hasn't aged then we bail out
early. This makes sure we decrement
ctx->current_pipeline_changes_since_flush so the next time we come to
flush a pipeline we will see a differing age.

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

cogl/cogl-framebuffer.c

@@ -257,6 +257,7 @@ _cogl_clear4f (unsigned long buffers,
           /* Make sure the ColorMask is updated when the next primitive is drawn */
           ctx->current_pipeline_changes_since_flush |=
             COGL_PIPELINE_STATE_LOGIC_OPS;
+          ctx->current_pipeline_age--;
         }
     }
 
@@ -1528,6 +1529,7 @@ cogl_framebuffer_set_color_mask (CoglFramebuffer *framebuffer,
   /* Make sure the ColorMask is updated when the next primitive is drawn */
   ctx->current_pipeline_changes_since_flush |=
     COGL_PIPELINE_STATE_LOGIC_OPS;
+  ctx->current_pipeline_age--;
 }
 
 gboolean