Blender Git Loki

Fix T81484: Weight/Vertex paint with mirror and viewport clipping does not update stroke on initial side

Issue introduced in rB4f616c93f7cb.

Issue here is that the the `StrokeCache` `mirror_symmetry_pass` is still
in its previous state when entering
`wpaint_do_symmetrical_brush_actions`.
For the initial stroke this means that the (wrong) cache
`mirror_symmetry_pass` ends up in SculptBrushTest `mirror_symmetry_pass`
as well and thus the clipping test in `sculpt_brush_test_clipping` will
fail.
[ This one flips the coords to test against clipping according to (now
wrong) `mirror_symmetry_pass` ]

Solution seems simple: just ensure we start of with a
`mirror_symmetry_pass` of zero in `wpaint_do_symmetrical_brush_actions`
for the initial stroke.
Same thing is done for vertex paint as well.

Maniphest Tasks: T81484

Differential Revision: https://developer.blender.org/D9268