Disable also NV_framebuffer_blit on ES2.

Until we have proper extension loader implemented (caused
FramebufferGLTest to assert).

On a related note, NVidia drivers 334.21 support *a lot* of ES2
extensions. Wow.
Remove redundant ifdefs and docs from desktop-GL-only code.
Doc++
Multisample textures don't have mip levels.

Another victim of copy-paste, which was never tested in practice because
of incomplete implementation.
Multisample textures also won't have any image setting functions.
Added MultisampleTexture::setStorage().

With this function multisample texture implementation is pretty much
done. Also implemented all the tests.
Compressed formats are not available for multisample textures.