Add support for generalized samplers, OpenGL interop, etc.

This allows us to handle stuff like sampler2DRect or samplerExternalOES etc. transparently. As an aside, we also properly handle e.g. usampler2D (for UINT textures) as part of the same refactor.

Requires some more API breaks (especially with regards to the PL_SHADER_SIG_SAMPLER2D, but since these APIs are relatively new it shouldn't bite any users in practice. This should hopefully give us all the tools VLC needs.