Add backend-specific resource info functions.

[floooh]

Those functions would allow to get the backend-API-specific resource objects wrapped by sg_buffer, sg_image, etc...

New structs and functions:

sg_[api]_[type]_info sg_query_[api]_[type]_info(handle)

Where [api] is [d3d11, gl, metal, wgpu] and type is [buffer, image, sampler, shader, pipeline, pass].

TODO:

• [x] d3d11
• [x] metal
• [x] gl
• [x] wgpu