Blender Git Loki

Fix T39279 Vertex paint modes do not work well with mirror modifier

Issue here is that coordinates used for projection would not fit the
closest candidate. So it was possible to paint against the mirrored part
of the mesh, which would get clipped on reprojection.

Fix avoids reprojection by storing squared distance, and screen space
coordinates, which we will need again later anyway. Also we now always
paint against the closest vertex always. This is ensured because squared
distances for uninitialized vertices will be MAXFLOAT, thus failing the
strength test.