Fix #105339: grease pencil selection can toggle object selection

Grease Pencil (when not in object mode) implements its own selection
opertor. This operator (`gpencil.select`) returns
`OPERATOR_PASS_THROUGH`, then falls though to `view3d.select` which can
toggle object selection (when using shift-click picking).

Removing `OPERATOR_PASS_THROUGH` would fix the object toggling, but this
was added in 62c73db734 with good reason (the tweak tool would not
work then).

Now prevent `view3d.select` from acting on Grease Pencil (when not in
object mode).

NOTE: longer term we could have grease pencil use view3d.select to avoid having to add these awkward exceptions

Pull Request #105342

source/blender/editors/space_view3d/view3d_select.cc

@@ -2992,6 +2992,15 @@ static int view3d_select_exec(bContext *C, wmOperator *op)
   Object *obedit = CTX_data_edit_object(C);
   Object *obact = CTX_data_active_object(C);
 
+  if (obact && obact->type == OB_GPENCIL && GPENCIL_ANY_MODE((bGPdata *)obact->data)) {
+    /* Prevent acting on Grease Pencil (when not in object mode), it implements its own selection
+     * operator in other modes. We might still fall trough to here (because that operator uses
+     * OPERATOR_PASS_THROUGH to make tweak work) but if we dont stop here code below assumes we are
+     * in object mode it might falsely toggle object selection. Alternatively, this could be put in
+     * the poll funtion instead. */
+    return OPERATOR_PASS_THROUGH | OPERATOR_CANCELLED;
+  }
+
   Depsgraph *depsgraph = CTX_data_ensure_evaluated_depsgraph(C);
   ViewContext vc;
   ED_view3d_viewcontext_init(C, &vc, depsgraph);