Optimize Workbench performance so it's on par with the previous
implementation.
Most of these changes are barely noticeable on powerful GPUs,
but can cause a notable performance improvement on old or low-end
hardware.
* Avoid unnecessary texture copies and draw directly to the viewport
textures.
* Optimize-out depth/stencil reads, using stencil testing instead.
* Avoid using `Texture::clear` and use framebuffer clears instead.
* Avoid framebuffer state changes (always use the same attachments).
* Avoid constant variation of acquired `TextureFromPool`s.
Fix#113010
Pull Request: https://projects.blender.org/blender/blender/pulls/113251
fe39456ba5