gn: fix fiddle

- __SK_FORCE_IMAGE_DECODER_LINKING makes sure the PNG encoder is linked
  - updated path to make working with sample draw.cpp easier.

No public API changes.
TBR=reed@google.com

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2202353002

Review-Url: https://codereview.chromium.org/2202353002

BUILD.gn

@@ -308,7 +308,9 @@ executable("fiddle") {
   deps = [
     ":skia",
     ":skia.h",
+    ":tool_utils",
   ]
+  testonly = true
 }
 
 template("test_lib") {

include/images/SkForceLinking.h

@@ -5,6 +5,8 @@
  * found in the LICENSE file.
  */
 
+#include "SkTypes.h"
+
 /**
  *  This function's sole purpose is to trick the linker into not discarding
  *  SkImageDecoder subclasses just because we do not directly call them.

tools/fiddle/draw.cpp

@@ -12,7 +12,7 @@
 #include "fiddle_main.h"
 DrawOptions GetDrawOptions() {
     // path *should* be absolute.
-    static const char path[] = "../../resources/color_wheel.png";
+    static const char path[] = "resources/color_wheel.png";
     return DrawOptions(256, 256, true, true, true, true, path);
 }
 void draw(SkCanvas* canvas) {

tools/fiddle/fiddle_main.cpp

@@ -7,6 +7,9 @@
 
 #include <stdio.h>
 #include <stdlib.h>
+#include "SkForceLinking.h"
+
+__SK_FORCE_IMAGE_DECODER_LINKING;
 
 #include "fiddle_main.h"