Fix skincolor-related memory leak

Currently, any change to a skincolor's ramp will cause the entire translation colormap cache to be cleared. Since this doesn't actually free the cached colormaps (since they're freed when PU_LEVEL blocks are purged), they remain in memory. Furthermore, animated skincolors take advantage of the fact that skincolor ramps are R+W and modify them often, worsening the issue and leading to a noticeable, continuous increase in memory usage.