1. When adding one pixel border to UV islands prefer grid directions.
This is more logical as it means neighbor pixels will commonly
share a border, opposed to just corners.
2. Don't subtract 0.5 when converting float UVs to int in computing
adjacency across a UV seam. Pixels cover a square with corners
at int positions and adding/subtracting 0.5 is only for dealing
with the center point of a pixel.
3. Use the neighbour_pixel field from the correct pixel, and check
it's not back to the origin point.
4. In the connected UV case, ensure that the returned index actually
refers to a valid active pixel.
This fixes paint spread not traversing some UV seams, while at the
same time spreading to random unrelated points on other seams.
The first problem is primarily fixed by 2, while the second one
is addressed by item 4.
Differential Revision: https://developer.blender.org/D2261
9a66d0ad1b