Mesh Optimization, Initial Steps

Since releasing Blender 2.8x there have been performance regressions with mesh editing that haven't been addressed.

While we were aware of these problems, early on a lot of time was spent investigating & fixing bugs
just to get feature parity with 2.7x.
Later on some improvements were made and Blender seemed more or less
usable for artists at the Blender institute where mesh editing performance wasn't such a high priority.

Having said this, artists in the community have been asking for improvements in this area
(see devtalk thread https://devtalk.blender.org/t/17415 ).

This kind of development needs focused time to investigate issues try, different solutions etc,
it's not so easy to fit this between regular development tasks.

So we're setting aside some time to improve the situation with a 6-8 week sprint,
with 3 developers (Germano Cavalcante, Jeroen Bakker and myself).

Recently we have been investigating bottlenecks in more detail,
with a focus on edit-mesh identifying areas to focus on.
and we're confident we can make significant improvements to performance.

In brief, the main bottlenecks are uploading data to the GPU as well as
redundant data-duplication and GPU-data rebuilding that can be skipped entirely.

For details see:

- Proposal for Mesh Optimization Project developer.blender.org/T88044
- Edit-Mesh Performance Overview https://developer.blender.org/T88021
- GPU: Mesh Drawing Performance https://developer.blender.org/T87835

... not so fast!

While we should be able to make gains with mesh-editing,
it's possible users with complex files won't notice much difference.

For example, files where subdivision-surface is the bottleneck
require development specifically to optimize OpenSubdiv.
In general files with heavy use of modifiers may not see much in the way of overall speed improvements,
since that's not our focus at the moment.

So for this sprint we keep our focus on areas we can get maximum benefit from,
in the future we can look into improvements in other areas.