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

source/blender/editors/interface/interface_handlers.c

@@ -1086,6 +1086,9 @@ static void ui_multibut_add(uiHandleButtonData *data, uiBut *but)
   mbut_state = MEM_callocN(sizeof(*mbut_state), __func__);
   mbut_state->but = but;
   mbut_state->origvalue = ui_but_value_get(but);
+#  ifdef USE_ALLSELECT
+  mbut_state->select_others.is_copy = data->select_others.is_copy;
+#  endif
 
   BLI_linklist_prepend(&data->multi_data.mbuts, mbut_state);