ability to build combinations of shaders

this is important for things like controlling mesh deformation using bones, and calculating this on the GPU. it's faster to calculate the mesh deformation on the GPU, but if you also want to use a shader to render the mesh, right now you would have a problem because only one shader would run.

for the skinning example above, the simplest version of this might just be loading separate vertex and fragment shaders. a more advanced version would allow multiple shaders to be evaluated in a single pass, for the vertex or fragment shaders.

No files, tests, or entry points are named. Start by locating the shader-loading and single-pass rendering entry points, then define how separate vertex and fragment shaders and multiple shaders per stage should combine; done means GPU mesh deformation and mesh rendering can run together.