NEW FEATURE: Multi-vertex merge. If, at the start of an operation, there is an existing selection of vertices, if the user clicks and drags on one of those vertices, then all selected vertices will be merged at the final location when the mouse button is released.
There is an add-on preference to enable/disable this feature (enabled by default).
Added vertex and fragment shaders and a dashed line class to support the new mode.
NEW FEATURE: If there was an existing selection of vertices or edges at the start of an operation and the user cancels with Right Click or Esc, then the selection is restored.
Got rid of distinct selection modes for a generic sel_mode with different states.
Upgraded and simplified the find_center function to work with many points, not just two.
sel_modewith different states.find_centerfunction to work with many points, not just two.