Blender Git Loki

Fix T78959: Show current frame indicator when interface is locked

When the playhead drawing moved to an overlay, a check was added to keep
it from drawing with a locked interface. This is necessary for some overlays,
but not this one, so this removes the check, making it the responsibility of
the editor.

A context function is added to make that check easier in the future.

Differential Revision: https://developer.blender.org/D8313