This was caused by an out-by one error when setting up the skin: if cmap
was 0 then the gl_skin struct would be taken from index -1 of the array and
thus cause all sorts of grief.
GL sometimes crashes when building skins. This probably isn't the correct
fix (finding the situation where fb->tex can become NULL despite fb being
non-null is), but it does kill the segfault. Luckily, this is git and this
commit can just be reverted when the real fix shows up. :)
In the end, it turns out this is the correct fix for the gl seg on
overkill, because build_skin will correctly use the fully setup player skin
if the glskin doesn't have a texture associated with it.
If the client receives a skin updated message from the server before having
loaded the player model (shouldn't happen, but some servers have very
strange programmers), no skin data is avaible for updating, so just bail
out.
Where possible, symbols have been made static, prefixed with gl_/GL_ or
moved into the code shared by all renderers. This will make doing plugins
easier but done now for link testing.
