Fix bug in OpenGL encore colormaps

This fixes wrong encore mode colors in OpenGL in Virtual Highway and Paradise Hill and various other maps.

Wrong kind of values were passed into NearestColor, resulting in encore colormaps getting the wrong colors. (mostly red and black)

The bugfix was discovered by Guil, and I cleaned up the code a bit further to use RGBA_t.

changed the description

merged

mentioned in commit f7e74e88

mentioned in merge request !183 (merged)