Blender Git Loki

Fix panning with Lock-to-Selection and no selection in Clip Editor

Rather self-explanatory. Never worked since the initial implementation.
It is possible to preserve lock-to-selection option with no selection
nowadays (since the fix for T84850). So now the Lock-to-Selection option
is fully under user control. Surely, the panning and zooming is also
properly supported now in the described scenario.

Differential Revision: https://developer.blender.org/D10226