Accumulating allocates previous colors (which are used in
`do_vpaint_brush_blur_XXX`).
The actual problem here was that the state of `brush_use_accumulate`
was not consistent across the lifetime of strokes.
Vertexpaint was doing the allocation in `vertex_paint_init_stroke`
(**before** `update_cache_invariants` where the mode gets changed to
`BRUSH_STROKE_SMOOTH` etc.), so here it still seemed we would use
accumulation, whereas later (after internally switching the tool/brush)
this was not the case anymore, leading to wrong behavior of
`do_vpaint_brush_blur_XXX`.
So now move the allocation to `init_session_data` (same as for
weightpaint) to make sure all codepaths have a consistent state of
`brush_use_accumulate`.
NOTE: this was made more obvious since 6de6d7267f added SHIFT-blurring
to the keymap
Pull Request: https://projects.blender.org/blender/blender/pulls/123778
d4f5d4a6f6