drm_hwcomposer: Add GetOrderLayersByZPos method

The GetOrderLayersByZPos copies an ordered list of layer pointers.

Signed-off-by: Matvii Zorin <matvii.zorin@globallogic.com>
diff --git a/DrmHwcTwo.h b/DrmHwcTwo.h
index bcd7eb1..a0d8cdd 100644
--- a/DrmHwcTwo.h
+++ b/DrmHwcTwo.h
@@ -174,6 +174,7 @@
     void RegisterRefreshCallback(hwc2_callback_data_t data,
                                  hwc2_function_pointer_t func);
     HWC2::Error CreateComposition(bool test);
+    std::vector<DrmHwcTwo::HwcLayer *> GetOrderLayersByZPos();
 
     void ClearDisplay();