• Added code to correctly detect 15-bit highcolor vs 16-bit highcolor
  display format support, despite SDL falsely indicating 16-bit
  color in both cases, which fixes incorrect colors on 15-bit display
  modes.
• Fix up VGA DAC/palette mapping to display the emulator screen
  properly on 16-bit true color displays. Fixed off-by-one shift
  mistake in the DOSBox-X render scaler code that caused
  green and purple tints to colors on 16-bit true color displays.