Use const access to snapshot from OpenGLRenderer

Additionally, move clipping methods to StatefulBaseRenderer

Change-Id: Iff232bf16fc1ad3b7d89493da6d8915db7bc5e4f
diff --git a/libs/hwui/LayerRenderer.cpp b/libs/hwui/LayerRenderer.cpp
index 35fd46e..0b38c4d 100644
--- a/libs/hwui/LayerRenderer.cpp
+++ b/libs/hwui/LayerRenderer.cpp
@@ -117,7 +117,7 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 Region* LayerRenderer::getRegion() const {
-    if (getSnapshot()->flags & Snapshot::kFlagFboTarget) {
+    if (currentSnapshot()->flags & Snapshot::kFlagFboTarget) {
         return OpenGLRenderer::getRegion();
     }
     return &mLayer->region;