Bugfix [#33974] Bone roll flips 180 degrees when extruding bones vertically

For example, if you're making a chain of bones (e.g. for a spine) by extruding
the tip joint of an initial bone, the bone rolls would be: 0 (for the initial
bone), 180, -180, 180, -180, etc. This has the undesirable effect of causing
B-Bones to twist to match the roll values at the other end of the bone.

The fix here seems to improve the situation in this case: bone roll values don't
flip or change anymore (in fact, the bone axes stay perfectly aligned now, as
they should). It also doesn't seem to cause any problems in other common cases I
checked.




git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54061 954f8c5b-7b00-dc11-b283-0030488c597c