If app requested <= 16 color depth and there is a 24-bit config available,
favor that. This fixes things that quietly expect to get truecolor output
but don't request it (...like SDL's render api...) and things that are
probably requesting 16-bit color as a fallback but expecting reasonable
systems to give them full depth.
Specifically, this fixes Life is Strange on Wayland, which uses the latter
approach, and anything using SDL_Render on Wayland, which uses the former.
Fixes#4056.
Fixes#4132.
354cabd4a7