Support a separate texture matrix for the background layer.

Support a separate texture matrix for the background layer.

Example shader stages using it:

{
	map textures/savdm6ish/#lava1.jpg
}
{
	map textures/final_rage/lava.blend.tga
	tcMod rotate 50
	tcMod scale 2 2
	tcMod scroll 0.05 0.05
	alphaGen vertex
	blendFunc GL_SRC_ALPHA GL_ONE_MINUS_SRC_ALPHA
}

NOTE: you still have to specify alphaGen vertex, or DP will not use the
secondary stage! To allow static mixing of two textures, the second texture
currently must have its own alpha channel.

Stuff also possible with this: detail textures...

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@9042 d7cf8633-e32d-0410-b094-e92efae38249