change resources returned to user to handles

[shakesoda]

this would make TFX_INVALID_* not spew warnings anymore, reduce rebuild times when modifying tinyfx (fewer header changes), and generally make the header a bit leaner.

some new functions that return internal resource info (texture width, height, mips, format...) would be needed and canvas reconfigure would need some rethinking, but most types would change to just struct { uint handle; }

as a handy side effect this would also be needed for any unspecified future port (vulkan?)