[cogl debug] --cogl-debug=rectangles now outlines all cogl rectangles

[cogl debug] --cogl-debug=rectangles now outlines all cogl rectangles

This makes the #if 0'd debug code that was in _cogl_journal_flush_quad_batch
- which we have repeatedly found usefull for debugging various geometry
issues in Clutter apps - a runtime debug option.

The outline colors rotate in order from red to green to blue which can also
help confirm the order that your geometry really drawn.

The outlines are not affected by the current material state, so if you e.g.
have a blending bug where geometry mysteriously disappears this can confirm
if the underlying rectangles are actually being emitted but blending is
causing them to be invisible.