debug: Adds COGL_DEBUG=winsys option

Although we internally had a COGL_DEBUG_WINSYS enum we weren't providing
a way to enable that via the COGL_DEBUG environment variable. This adds
a "winsys" option that can be used to enable printing of winsys debug
notes.

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

cogl/cogl-debug.c

@@ -55,7 +55,8 @@ static const GDebugKey cogl_log_debug_keys[] = {
   { "offscreen", COGL_DEBUG_OFFSCREEN },
   { "texture-pixmap", COGL_DEBUG_TEXTURE_PIXMAP },
   { "bitmap", COGL_DEBUG_BITMAP },
-  { "clipping", COGL_DEBUG_CLIPPING }
+  { "clipping", COGL_DEBUG_CLIPPING },
+  { "winsys", COGL_DEBUG_WINSYS }
 };
 static const int n_cogl_log_debug_keys =
   G_N_ELEMENTS (cogl_log_debug_keys);