Make onPrepareDraws const

gm/beziereffects.cpp

Index: gm/beziereffects.cpp
diff --git a/gm/beziereffects.cpp b/gm/beziereffects.cpp
index ba163a3840f77145033512b8948b74b7fd43f433..5167b770134b6023ea5dbd851a9ddc37a20f2364 100644
--- a/gm/beziereffects.cpp
+++ b/gm/beziereffects.cpp
@@ -70,7 +70,7 @@ private:
         return &fGeometry;
     }
 
-    void generateGeometry(Target* target) override {
+    void generateGeometry(Target* target) const override {
         QuadHelper helper;
         size_t vertexStride = this->geometryProcessor()->getVertexStride();
         SkASSERT(vertexStride == sizeof(Vertex));
@@ -470,7 +470,7 @@ private:
         return &fGeometry;
     }
 
-    void generateGeometry(Target* target) override {
+    void generateGeometry(Target* target) const override {
         QuadHelper helper;
         size_t vertexStride = this->geometryProcessor()->getVertexStride();
         SkASSERT(vertexStride == sizeof(Vertex));