Palette index 255/Pure cyan is invisible in OpenGL
Most obvious as a cyan-coloured Knuckles, but also (non-)present on other cyan-coloured things (including A-rank emblems on the statistics menu), it seems that palette index 255 or pure cyan is transparent in the OpenGL renderer, even when just due to palette-remapping. The Software renderer is unaffected, with palette index 255 being rendered correctly.
This applies to both the current master branch (27285365) and the current next branch (2a9a3b97), though not the officially-released v2.2.0 (69c08f93).
I've compiled the game locally using MinGW64 for all 3 of the above commits to test, with Windows 10 (64-bit) as both the compiler and the output target, and I use an NVIDIA GPU for playing the game.
