Add cl_dyntexture.h and .c (don't need to be added to the makefile or vc project...

Add cl_dyntexture.h and .c (don't need to be added to the makefile or vc project yet). They expose a simple interface to manage dynamic textures.
It supports getting the current texture handle for a dynamic texture path, linking a dynamic texture path to a texture handle and unlinking a dynamic texture path.
This is pretty transparent and it could be used for arbitrary dynamic textures (even ones that aren't updated but only linked dynamically into the game world - alias texture names).
This works for both skinframes and cachepics. Still contains lots of todos.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@7713 d7cf8633-e32d-0410-b094-e92efae38249