vrend/shader: add support for separable shaders
This currently slightly improves performances in some games that make heavy use of separable shaders, but has a few problems to be solved before we can consider merging it.
TODO:
- Deal with conflicting
layout (location=X)
qualifiers. - Test more, be sure that the performance improvement is worth merging.
- Squash WIP commits after issues are resolved.
cc @gerddie
Edited by Italo Nicola