Support for setting a separate color per model instance

This information comes into shaders as 'universalColorAttrib', if
desired.
Fixed a few more bugs in per-instance color support.

Per-instance color is now handled separately from vertex color and
universal color.  Vertex color still picks up universal color, if no
vertex color data is provided.  (TODO:  Clean that up)
Fix a spot where a BatchElement wasn't being cleared.

Didn't actually cause problems, since every field was being reset in
this function, but for future-proofing.
Adds support for instanced drawing of line strips and line lists
Merge branch 'instance_color'