And further clean up skin api.
It turns out that skin functions must all be in the render libs, and
this results in Skin_Set (was Skin_SetSkin) needs to be accessed via a
function pointer rather than directly :(
This takes care of the double free and also cleans up a lot of the skin
api. However, the gl renderer lost top/bottom colors (for now). Vulkan
skins still don't work yet.