Blender Git Loki

Fix T55632: USE_DRAG_MULTINUM and USE_ALLSELECT not working well together

'is_copy' was not set correctly on all uiButMultiState (it was done once
for uiHandleButtonData), resulting in 'delta' being used on some indices
of the array and not others in `ui_selectcontext_apply`.

Maniphest Tasks: T55632

Differential Revision: https://developer.blender.org/D6201