Separate load/store actions for render passes.

[floooh]

This brings render pass handling closer to Metal / WebGPU:

• sg_pass_action now has separate store actions
• sg_pass now has separate resolve-target images
• filled some gaps in sg_begin_pass() re image types
• msaa images no longer have an implicit resolve-texture, instead this needs to be a separate sg_image
• more detailed validation in sg_make_image, sg_begin_pass and sg_apply_bindings

TODO:

• rename sg_image_desc.render_attachment back to sg_image_desc.render_target (Metal calls the flag 'render target' while WebGPU calls it 'render attachment', we'll just stick with Metal in this case)
• ...also in samples
• fix d3d11 samples
• dummy backend
• fix tests
• add tests for sg_make_image validation checks
• add tests for sg_make_pass validation checks
• update documentation comments
• update readme and changelog
• fix language bindings samples
• investigate and fix the Rust bindings clear-color bug (the clear sample background color is green-cyan instead of yellow-red) => nvm, the C library just wasn't recompiled, deleting the target directory fixed the problem (still strange, looks like cargo's dirty-checks don't work for compiling C libraries?)