sokol_imgui.h: proper support for user images with samplers.

[floooh]

Adds the following to sokol_imgui.h:

• simgui_image_t which groups a sokol-gfx image and sampler
• simgui_image_desc_t desc struct to create an simgui_image_t
• simgui_make_image() to create an simgui_image_t object
• simgui_destroy_image() to destroy an simgui_image_t object
• simgui_imtextureid() has been changes to take an simgui_image_t handle instead of sg_image
• simgui_desc.image_pool_size
• simgui_desc.logger: added logging support
• allocation failure now results in a log-panic
• added an 'init_cookie' to make sure that API functions are only called between setup/discard

TODO:

• [x] fix sokol_gfx_imgui.h
• [x] fix existing sokol samples
• [x] add a new sokol sample to demonstrate using custom images with sokol_imgui.h
• [x] add documentation
• [x] changelog + readme