[next] Colormap caching memory leak

Calling v.getColormap in HUD hooks in next originates a leak. The __gc metamethod introduced to colormap userdata by !959 (merged) is never called for some reason, so unreferenced colormaps can keep piling up as long as the HUD is displayed. Map changes can still free them, though.

This script can be used to verify the issue. It happens in both renderers, but OpenGL will run out of memory much faster. colormaptest.lua