Blender Git Loki

Moved code for modifier evaluation out of DerivedMesh.c.

These files contained a confusing mix of DM code (for manipulating DMs
and various utility functions relating to these types), and the code
for evaluating object modifier stacks. These are two very distinct
areas and putting them in a single huge file doesn't promote code
clarity at all.

This commit only moves code, but does not alter any of it. This is
important for log continuity, so history of specific functions can be
clearly traced back!