Palette additions (SGB + original GB hardware) and improvements to auto GB colourisation

[jdgleaver]

I recently stumbled upon a nice fork of Gambatte (https://github.com/TRIFORCE89/Gambatte-Core) which adds Super Game Boy palettes and automatic selection of the 'best' (most colourful) palette when colourising original Game Boy games. This pull request adds the same functionality to the libretro core (all credit goes to @TRIFORCE89, the original author).

The new palettes are exposed via the 'Internal Palette' core option, and the 'GB Colorization' option has been modified as follows:

• auto: now selects a palette with the following order of preference:

  1. SGB, if more colourful than GBC

  2. GBC, if more colourful than SGB

  3. SGB, if game has no predefined GBC palette

  4. Value set by 'Internal Palette' core option, if game has neither SGB nor GBC predefined palettes

• gbc: forces GBC colourisation, falling back to 'GBC - Dark Green' (hardware default) if game has no predefined palette

• sgb: forces SGB colourisation, falling back to 'SGB - 1A' (hardware default) if game has no predefined palette

While I was editing palettes, I thought I might as well complete the set. To that end, I have also added three 'GB' palettes designed to mimic the display characteristics of the original Game Boy hardware revisions: DMG, Pocket and Light.

[inactive123]

Great addition! Thanks a lot.