Support Sampler binding in the shader.

In the cases where we support the binding layout keyword, use it for more than binding UBO location.
This changes it so it is supported for samplers as well.

Instances when this is enabled is if a device supports GL_ARB_shading_language_420pack, or if it supports GLES 3.10.
We just did use samp0 everywhere and this code was shared with d3d9, so I didn't want to touch it. As d3d9 is removed, this is now ogl only.