Blender Git Loki

DRW: Add instance buffer manager.

This manager allows to distribute existing batches for instancing
attributes. This reduce the number of batches creation.
Querying a batch is done with a vertex format. This format should
be static so that it's pointer never changes (because we are using
this pointer as identifier [we don't want to check the full format
that would be too slow]).

This might make the original Instance Data manager useless but it's currently used by DRW_object_engine_data_ensure().