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.
