When using multiple materials in a single mesh the most time is spend in
counting the offsets of each material for the sorting.
This patch moves the counting of the offsets to render mesh data and
caches it as long as the geometry doesn't change.
This patch doesn't include multithreading of this code.
Reviewed By: mano-wii
Differential Revision: https://developer.blender.org/D11612
174ed69c1b