Otherwise, SDL_PIXELFORMAT_BGR24 is reported as having alpha, because
its SDL_ARRAYORDER_BGR pixel order uses the same integer value as
SDL_PACKEDORDER_RGBA, since we weren't checking the pixel type to
differentiate.
Fixes Bugzilla #2977.
An existing hint lets apps that don't need the timer resolution changed avoid
this, to save battery, etc, but this fixes several problems in timing, audio
callbacks not firing fast enough, etc.
Fixes Bugzilla #2944.
Ryan C. Gordon
Sam Lantinga