sokol_gfx.h: allow multisampled images as resource bindings

[floooh]

Allows to fetch samples in shader from multisampled textures (for instance to allow custom-resolve render passes).

Not supported on WebGL2/GLES3 because no of missing support for multisampled texture objects, and also not on macOS because it just doesn't seem to work (even though the required function exist, but only in gl3ext.h, not in gl3.h).

Related ticket: #1142

• changelog entry
• integrate new custom resolve sample into sample webpage
• update docs (sg_features.msaa_image_bindings)
• add a coverage visualization to the custom resolve sample
• test:
  • macOS + Metal
  • macOS + GL
    • crash: Assertion failed: (glGetError() == 0), function _sg_gl_cache_bind_texture_sampler, file sokol_gfx.h, line 8195. simply disable msaa texture code path on macOS+GL, not worth the hassle
  • iOS + Metal
  • iOS + GLES3 (not supported)
  • Windows + D3D11
  • Windows + GL
  • Linux + GL
  • WebGL2 (not supported)
  • WebGPU
  • Android (not supported)