opengles emulator: replace the readback at end of frame with blit

That replaces the readback to host we do at the end of each frame
with two blits on the GPU, one to copy the pixels into a texture
and another to render the texture to another in order to flip
the image.

Change-Id: I7e0e10493d38944d0b613e245023f34236d3dfc4
4 files changed