Planning: Resource binding cleanup

[floooh]

This would make sense after the storage-buffer update:

General idea is to allow more flexibility when mapping the sokol-gfx 'bind slot convention' to the backend API bind slots.

• Get rid of shader stages in sg_bindings:

typedef struct sg_bindings {
    uint32_t _start_canary;
    sg_buffer vertex_buffers[SG_MAX_VERTEXBUFFER_BINDINGS];
    int vertex_buffer_offsets[SG_MAX_VERTEXBUFFER_BINDINGS];
    sg_buffer index_buffer;
    int index_buffer_offset;
    sg_image images[SG_MAX_IMAGE_BINDINGS];
    sg_sampler samplers[SG_MAX_SAMPLER_BINDINGS];
    sg_buffer storage_buffers[SG_MAX_STORAGEBUFFER_BINDINGS];
    uint32_t _end_canary;
} sg_bindings;

The sg_shader_desc needs to provide a mapping from those unified "sokol-gfx bindslots" to shader stages and (probably backend-API specific) concrete bind slots (typically generated by sokol-shdc).

• Q: also get rid of the 'implicit' backend-specific bind slot indices? This means the sg_shader_desc bind slot-mapping needs backend-specific slot indices (which is probably more transparent to the user)

• while at it also get rid of the stage param in sg_apply_uniforms() (and provide a mapping in sg_shader_desc instead for backends which need this)

• TL;DR: SG_SHADERSTAGE_VS/FS should only show up in sg_shader_desc.