* Refacor uniforms API:
- More complete and consistent
- Type-safety
- Usage correctess assertions missing for now, planned
* Redesign texturing API: texunits gone, assign textures to sampler uniforms directly
r_texture_create now allocates memory and returns an opaque Texture
pointer; similar changes to the other renderer APIs will follow.
* Framebuffers: make _create return an opaque pointer, add debug label APIs (unused for now)
* opaque pointers and debug label APIs for vertex arrays and buffers
* fix null renderer
* Refactor glsl preprocessing into an independent module
* Separate shader resource management from renderer backend
This makes it possible to add more shading languages and/or include a
transpiler, which will be useful for the GLES backend.
* refactor r_clear into a stateless API
* add r_texture_clear API; fix gl33_framebuffer_clear
* Replace deprecated glsl_objects with objects in all *.prog files
* fix missing texture_clear implementation in null renderer
* remove some dead code in null renderer
* fix GLES segfault
* GLES 3.0 actually has glVertexAttribDivisor
* Query GL for supported GLSL versions (preparing to add shader transcompilation)
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