Evas GL: Kill runtime warnings caused by auto fallback

Evas GL: Kill runtime warnings caused by auto fallback

The previous commit modifies the concept of direct rendering
vs. indirect rendering, so some runtime checks (in debug mode
only) will fail.

This commit introduces two new engine functions:
 - gl_get_pixels_pre
 - gl_get_pixels_post

The latter will be used in a later patch for optimization.