add /img/n/m endpoint to skiaserve

tools/skiaserve/Request.h

Index: tools/skiaserve/Request.h
diff --git a/tools/skiaserve/Request.h b/tools/skiaserve/Request.h
index 60a59a7b1cd82a9b935f9a064d106f6a13ddf1bd..0b974e74237460e385a36f10126f8c6b6bfc208a 100644
--- a/tools/skiaserve/Request.h
+++ b/tools/skiaserve/Request.h
@@ -29,7 +29,8 @@ struct UploadContext {
 struct Request {
     Request(SkString rootUrl); 
 
-    SkData* drawToPng(int n);
+    // draws to skia draw op N, highlighting the Mth batch(-1 means no highlight)
+    SkData* drawToPng(int n, int m = -1);
     SkCanvas* getCanvas();
     SkBitmap* getBitmapFromCanvas(SkCanvas* canvas);
     bool enableGPU(bool enable);
@@ -57,7 +58,7 @@ struct Request {
     
 private:
     SkData* writeCanvasToPng(SkCanvas* canvas);
-    void drawToCanvas(int n);
+    void drawToCanvas(int n, int m = -1);
     SkSurface* createCPUSurface();
     SkSurface* createGPUSurface();
     GrAuditTrail* getAuditTrail(SkCanvas*);