video.c: Use ReGBA_IsRenderingNextFrame in update_scanline, cancelling the drawing if the next frame is not being rendered.

DSTwo, GCW Zero: Define ReGBA_IsRenderingNextFrame. On the GCW Zero, this optimises fast-forwarding because skipped frames are not rendered anymore.

sound.c: Define ReGBA_DiscardAudioSamples, which efficiently discards samples from the core's audio buffer.

GCW Zero: Downsample audio from 88200 Hz to 44100 Hz. Formerly, the SDL audio thread took 7-11% CPU. I expected it to take 4-6% CPU at 44100 Hz, but it takes less than a tenth a percent; 'htop' doesn't see usage from it ever.