Spinoff from #965 for a specific question.

From #965 (comment), D3D12 only allows float values for ClearRenderTargetView and D3D12_RENDER_PASS_BEGINNING_ACCESS_CLEAR_PARAMETERS. In order to make the behavior portable we have two options:

• Round the clear value to float in all backends before passing the value in (makes it possible, e.g., to clear to 0xffff_ff00 but not any value greater than that).
• Emulate the clear with a fullscreen quad in D3D12 (suggested here).