Fix EGLImage depth image test issues.

Fix EGLImage depth image test issues.

- Fix reversed depth compare in reference image generation.
- Set probe plane z coordinates in clip space.
- Use depth mask to avoid probe planes from altering the result.
- Clear fb color before rendering probe planes.
- Use proper threshold when reading a RGB4 fb.
- Initialize RBOs to a nice pattern instead of a single value.
- Don't render from images in multicontext cases before creating it.

Bug: 20637957
Change-Id: I158e51489ab8d562e74066b8549f5ee5a0d90cb7