Temporarily remove uses of RHI renderbuffers

OpenGL does not have renderbuffers until OpenGL 3.0 (contrast to
GLES which has them starting with 2.0). In order to allow people
with older GPUs to play the game for now (until ANGLE+GLES), I am
removing the use of renderbuffers in the game code. They aren't
needed because nothing actually needs a depth or stencil test yet.
Once HWR2 gets rolling, I'll add these back.

Fixes #1