sokol_gfx.h: add sg_query_[buffer|image|shader|pipeline|pass]_desc() functions.

[floooh]

Adds a set of functions for obtaining a (more or less complete) sg_*_desc struct of an existing resource. The structs may only be partially filled, any parameters which have not been kept around internally will be zeroed (but anything except sg_shader_desc structs is quite complete).

Related to issue #568

• sg_query_buffer_desc() + test
• sg_query_image_desc() + test
• sg_query_shader_desc() + test
• sg_query_pipeline_desc() + test
• sg_query_pass_desc() + test
• update docs
• update changelog