Blender Git Loki

Modifier: Add "Connected" mode to the weld modifier

Implement improvement from T73139 for merging along edges.
It is now called "Connected" mode, while the default is called "All".

With the recent performance improvement, the Connected Mode is in some
cases only double the speed than the usual merge all strategy but in
other cases it may be even faster. The bottleneck is somewhere further
down the line of merging geometry.

The motivation for this patch came from T80897, because the merging in
complex solidify is making it very slow.
Now merging can be removed from solidify without greater consequences,
as this is just a quicker and more advanced algorithm to do the same
thing that solidify currently does slowly.

Reviewed by: mano-wii, campbellbarton

Ref D8966