Add support for non-linear MD RGB values.

[GerbilSoft]

Previously, Gens/GS had an option to select a "color scale method": "raw" (naive bitshifting), "full" (better calculated so that white is 0xFFFFFF), and "full + S/H" (highlight adjusted to be correct). These were all removed in favor of just "full + S/H", since that's the only correct one.

http://gendev.spritesmind.net/forum/viewtopic.php?f=22&t=2188 has (relatively) newly-measured values from the MD's VDP that shows the RGB levels are actually non-linear. In particular, shadow values are higher than linear, and highlight values are mostly lower.

The "color scale method" option should be re-added with a better name and allow the user to select between linear or HW-measured color.