In case two neighbor buttons are very small, their total width (or height) can remain below
max authorized delta, and hence wrong side could be detected as 'common side' for the pair.
This is now fixed by checking both opposite sides at once.
Also, we expect buttons to have some width and height to be considered alignable now!
Took me two days to sort that out, grrrr!
Looks like UI code can produce widgets with same left/top positions (in very narrow space case).
Not nice, but we do not really care, UI becomes unusable way before we reach that point anyway.
This new code fixes a tons of issues with previous one, which basically was epic-failing
in many non-basic cases (especially mixed columns and rows with column-dominant layout).
It basically no more relies over order of buttons declaration in the uiBlock, instead it
finds and stores spatial neighbors and uses that data to compute needed stitching.
See code comments for details.
New code seems to be roughly ten times slower than old one (for complex grouped layouts),
that is, about a few microsecconds per alignment group - this remains reasonable.
Also, ui-align code becomming rather big in itself, it was separated in
own new `interface_align.c` file.
Reviewers: campbellbarton, severin
Reviewed By: campbellbarton, severin
Differential Revision: https://developer.blender.org/D1573
