WIP: Implement GL_ANGLE_framebuffer_blit
A strict GLES2 subset of EXT_framebuffer_blit
. Mesa supports the EXT version unconditionally, so the additional restrictions are added by checking explicitly for GLES 2.0.
Not quite done yet, need to write the check that z/s blits copy the entire buffer (post-scissor), and should quote the spec explicitly. But hey, it compiles.