wgl, d3d12: Fix winsys handling for apps requesting low color channel bit sizes
This piggybacks on the handling of the B5G6R5 format to add B4G4R4A4 (and R10G10B10A2 for completeness, though the frontend doesn't report this PFD currently, it should/could). Then updates the support query to report that these formats work, and fixes the d3d12 driver to start calling the callback.
Closes #9460 (closed)