Allow shaders to use embedded textures

[hemebond]

I want to be able to use the embedded textures when using shaders for Quake 1 maps, rather than having to duplicate the file outside. FTEQW supports this with the $diffuse variable, e.g.:

mytexture
{
	{
		map $diffuse
		tcMod stretch sin 1 1 4
	}
	{
		map $lightmap
		blendfunc GL_DST_COLOR GL_ZERO
		rgbgen identity
	}
}