sms_vdp: Improve Mode4 sprite rendering
sms_vdp: Improve Mode4 sprite rendering

The behaviour is now more like real hardware.

 * For each line, up to eight sprites are added to a sprite buffer
 * The first sprite added to this buffer that fills a pixel is the sprite
   that stays on that pixel. Implemented by traversing the buffer
   backwards.

This fixes the draw-order issue seen in Micro Machines, where all cars
appeared gray.

JoppyFurr/Snepulator,JoppyFurr/Snepulator,JoppyFurr/Snepulator,JoppyFurr/Snepulator,JoppyFurr/Snepulator,JoppyFurr/Snepulator