Disabled group-based method of material animation

We will soon be switching to the stage-based method for animating
all materials.

Define LIBDENG_OLD_MATERIAL_ANIM_METHOD to reinstate support for
this method of material animation.
Materials: Generate material layer animations from texture anim groups

Beginning to flesh out the new staged based mechanism for material
animation.

Interpret the game-registered texture anim groups when generating
materials from/for original game textures, reconstructing their
animations in terms of material's layer stages.

Todo: Implement interpretor logics for legacy group animation defs.
Todo: Reimplement smooth texture animation.
Todo: Reimplement support for flags such as PGF_GROUP.
Todo: Cleanup