For paint modes the selection and visibility were encoded in in the W
component of the normals. Separating the normals into a separate vertex
buffer, though it increases memory usage a bit in those paint modes,
means the status doesn't need to be computed outside of paint modes,
and the whole system becomes less convoluted and easier to optimize.
The particular optimization I have in mind is better caching of vertex
buffers, which gets much more complicated when the normals VBO contains
data like selection and visibility and depends on the paint mode.
Internally, a there are now three mesh draw batches that are specific to
the paint overlay. The GPU subdivision normals buffer is now just three
components rather than 4 components.
Pull Request: https://projects.blender.org/blender/blender/pulls/140156
4b4ed8bccc