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.
Yeah, sorry for this hack. Maybe we should replace samp0 with anything not pointing to a sampler unit?