Blender Git Loki

DRW: optimize mesh data extraction

Change extraction callbacks to take index ranges instead of calling them
for each mesh element (poly, loop, vert & edge).

This gives a minor overall performance gain in my tests, ~5% on average.

Details:

- Use typed parameter structs and macros for looping over elements.
Without this, changes to these callbacks is time consuming as changes
need to be made in many places.
- Avoid iterating over polygon-loops when iterating over polygons
is sufficient.
- Simplify logic to access adjacent loops for faster line extraction.
- Rename 'loop' iterators to 'poly' (as they take polygon ranges)
the iterator callbacks can operator on either polygon or loop data.
- Use term 'last' for the last index (inclusive),
use 'end' when this value (not inclusive).