When a glTexImage(GL_PROXY_TEXTURE_x) call generates an error there are
two possible outcomes:
1. Generate a regular GL error.
2. Don't generate a GL error.  Instead, zero-out the texture image's
   width, height, depth and format.

The spec isn't totally explicit about all this but in general the second
case happens when we exceed texture size/memory limits (that's the point
of proxy textures afterall).

Test passes with NVIDIA's driver.  Untested with AMD.  Mesa fixes/patches
are pending.