gl: Set is_clear flag to FALSE after map_to_image

In _cairo_gl_surface_map_to_image(), the image surface data has been
filled by glReadPixels, so is_clear flag should be set to FALSE.
Otherwise mapped image surface does not get drawn as it is presumed
clear and so returns true from nothing_to_do().

src/cairo-gl-surface.c

@@ -1121,6 +1121,7 @@ _cairo_gl_surface_map_to_image (void      *abstract_surface,
 	    free(row);
     }
 
+    image->base.is_clear = FALSE;
     return image;
 }