This is done because they don't have the same update frequency. UV can be
persistent even on geometry update (ex: skinned object) but tangents
can change if the normals change.
Also the name buffer per vbo was too small to contain all names.
For clarity sake, the batch cache now uses exclusively per Loop attributes.
While this is a bit of a waste of VRAM (for the few case where per vert
attribs are enough) it reduces the complexity and amount of overall VBO
to update in general situations.
This patch also makes the VertexBuffers filling multithreaded. This make
the update of dense meshes a bit faster. The main bottleneck is the
IndexBuffers update which cannot be multithreaded efficiently (have to
increment a counter and/or do a final sorting pass).
We introduce the concept of "extract" functions/step.
All extract functions are executed in one thread each and if possible,
using multiple thread for looping over all elements.
Reviewed By: brecht
Differential Revision: http://developer.blender.org/D5424
