Blender Git Loki

Fix vertex paint: color transform operators not respecting vertex paint
mask selection

for 'Invert', 'Levels', 'Hue saturation Value' and 'Bright/Contrast',
face mask was respected, but vertex mask wasnt...

Same code as done in 'Set Vertex Colors'.

reported in T69835

Reviewers: brecht

Maniphest Tasks: T69835

Differential Revision: https://developer.blender.org/D5783