add /img/n/m endpoint to skiaserve

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1755563003

Review URL: https://codereview.chromium.org/1755563003
diff --git a/tools/debugger/SkDebugCanvas.h b/tools/debugger/SkDebugCanvas.h
index d81d471..7379698 100644
--- a/tools/debugger/SkDebugCanvas.h
+++ b/tools/debugger/SkDebugCanvas.h
@@ -68,8 +68,9 @@
         Executes the draw calls up to the specified index.
         @param canvas  The canvas being drawn to
         @param index  The index of the final command being executed
+        @param m an optional Mth gpu batch to highlight, or -1
      */
-    void drawTo(SkCanvas* canvas, int index);
+    void drawTo(SkCanvas* canvas, int index, int m = -1);
 
     /**
         Returns the most recently calculated transformation matrix