Blender Git Loki

Fix T87406: Stroke color when using object color mode

The color of the strokes was shifted when using `Single Color` or
`Object Color`in Solid Display Mode. This was originally done so
that it was still possible to differentiate strokes and fills.

The fix allows for a completly flat shading by checking if the
Lighting Mode is set to `Flat`. If it is, then the colors are not
shifted for the strokes and everything will use the same color.

Reviewed By: antoniov, fclem

Maniphest Tasks: T87406

Differential Revision: https://developer.blender.org/D10957