Since Dear ImGui's ImTextureID is now guaranteed to be 64 bits, the sokol-gfx image+sampler pair can now be stashed directly into the ImTextureID instead of requiring a 'backing object'. This simplifies the sokol_imgui.h implementation drastically.

Also: in sokol_gfx.h, add granular sg_query_buffer_X() and sg_query_image_X() functions which allow to cheaply query buffer and image properties without having to return a big desc struct (might make sense to extend for samplers, but probably less useful for shaders, pipelines and attachments).

TODO:

• update inline documentation and comments
• update changelog
• merge Fixes for sokol_imgui.h ImTextureID simplification. sokol-samples#155