Blender Git Loki

Cleanup for mesh_calc_modifiers: Context and Iteration structs.

These gather most of the large number of local variables in that function,
in order to gain some clarity on their usage and scope.

Context is where immutable settings and global precomputed data is stored
during the computation. These values are initialized once and not touched
by modifiers.

Iterator struct holds the combined md/datamask pointers and some (hackish)
mutable data that gets updated during the process.

Where possible local variables have also been moved into narrower code blocks
to make the overall procedure less confusing.