Verify that *Texture::bind() doesn't emit any GL errors.

Can't verify much more at this time.
Save texture target along with ID in state tracker.

Makes later unbinding easier (i.e. we don't need to cycle through all
possible/supported targets and call glBindTexture(target, 0) with each.
Added *Texture::unbind().

Will be used in fallback implementation of multi-bind.
Support ARB_multi_bind for textures.
Fix (workaround) failing texture tests.

With ARB_multi_bind it is needed to associate the texture with some
target before calling glBindTextures(), otherwise the texture is
treated as invalid.
Shaders: use texture multi-bind functionality in Phong shader.