Even if multires in sculpt mode doesn't yet support color
attributes, we should at least upload white color to avoid
making everything black.
Also fixed a bug where multires PBVHs didn't have access to
their CustomData attribute layout, which PBVH draw needs.
Rewrite PBVH draw to allocate attributes into individual VBOs.
The old system tried to create a single VBO that could feed
every open viewport. This required uploading every color and
UV attribute to the viewport whether needed or not, often exceeding
the VBO limit.
This new system creates one VBO per attribute. Each attribute layout is
given its own GPU batch which is cached inside the owning PBVH node.
Notes:
* This is a full C++ rewrite. The old code is still there; ripping it out
can happen later.
* PBVH nodes now have a collection of batches, PBVHBatches, that keeps
track of all the batches inside the node.
* Batches are built exclusively from a list of attributes.
* Each attribute has its own VBO.
* Overlays, workbench and EEVEE can all have different attribute
layouts, each of which will get its own batch.
Reviewed by: Clement Foucault
Differential Revision: https://developer.blender.org/D15428
Ref D15428
