Blender Git Loki

WM: prevent drag events being continually tested

Click-drag events that weren't handled would continually be tested
for each mouse-motion event.

As well as being redundant, this added the overhead of querying
gizmos twice per motion event.

Now click-drag is only tested once when the drag threshold is reached.

This mitigates T87511, although the single drag test still causes
the snap gizmo to flicker.