sokol_letterbox.h and sokol_framebuffer.h #1483

Some recreational coding:

• add a sokol_letterbox.h header for fixed-aspect-ratio rendering (...and then integrate into chips)

• add a sokol_framebuffer.h header

  • various paletted and direct pixel formats
  • palette resolution in shader
  • allow pluggable CRT shaders
  • integrate into chips

• post-merge todo for sokol_letterbox.h:

  • port chips-test
  • port doom-sokol
  • port pacman.c (doesn't actually make sense)

Letterbox PR: #1491