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.