Drawing new geo across the symmetry line

[jlampel]

Similar to how we should respect the Clipping option in any existing mirror modifier on the retopology object (#1335), we should also use Clipping to determine whether or not we allow geometry to be created on the other side of the symmetry line if drawing across it in any of the insert tools.

Before, we had a "correct" side of the symmetry plane for users to draw on, but in Blender you can work on either side. What I think would work well is to consider whichever side the stroke starts on to be the "correct" side, and for any geometry that crosses to the other side, create verts directly along the symmetry line and remove any that are on the opposite side.

This is different from Blender's usual behavior, but it would be a big improvement over Blender's current tools.

Note: if this is implemented, I should add a Symmetry panel to the tool settings where users can easily manage the settings without digging through the modifier stack.