cogl debug: Adds glViewport call tracing

To aid in the debugging of Clutter stage resize issues this adds a
COGL_DEBUG=opengl option that will trace "some select OpenGL calls"
(currently just glViewport calls)

cogl/cogl-debug.h

@@ -45,7 +45,8 @@ typedef enum {
   COGL_DEBUG_FORCE_SCANLINE_PATHS = 1 << 14,
   COGL_DEBUG_ATLAS            = 1 << 15,
   COGL_DEBUG_DUMP_ATLAS_IMAGE = 1 << 16,
-  COGL_DEBUG_DISABLE_ATLAS    = 1 << 17
+  COGL_DEBUG_DISABLE_ATLAS    = 1 << 17,
+  COGL_DEBUG_OPENGL           = 1 << 18
 } CoglDebugFlags;
 
 #ifdef COGL_ENABLE_DEBUG