vertex and uniform declaration issues

[floooh]

• vertex layout definition should only happen in pipeline setup, not in shader setup (or only if required by backend API)
• there should be 2 ways to define the vertex component location, which one is used depends on rendering backend and shader language dialect:
  • by name: sg_pipeline_setup_named_attr()
  • by slot index: sg_pipeline_setup_indexed_attr()
• there must be several ways to describe uniform blocks in the shader setup:
  • per uniform, by name (traditional GL)
  • opaque uniform blocks (only useful with Metal, D3D11 or GL with shader-code-generation)
  • in general, shader setup should have different options depending on whether some sort of shader code generation is available, or shader setup needs to happen "manually"
• vertex layout definition should have an explicit offset