GPv3: Switch Direction operator

This Operator is similar to GPv2 GPENCIL_OT_stroke_flip to switch the stroke curve direction.

@@ -951,0 +968,4 @@
+        if (!ed::curves::has_anything_selected(curves)) {
+          return;
+        }
+        bke::MutableAttributeAccessor attributes = curves.attributes_for_write();

@@ -951,0 +973,4 @@
+        IndexMaskMemory memory;
+        IndexMask selected_curves = ed::curves::retrieve_selected_curves(curves, memory);
+
+        selected_curves.foreach_index([&](const int curve_index) {

@@ -951,0 +970,4 @@
+        }
+
+        IndexMaskMemory memory;
+        IndexMask selected_curves = ed::curves::retrieve_selected_curves(curves, memory);

@@ -951,0 +964,4 @@
+        bke::CurvesGeometry &curves = drawing.strokes_for_write();
+        
+        IndexMaskMemory memory;
+        IndexMask selected_curves = ed::curves::retrieve_selected_curves(curves, memory);