DM: Add support for reading a directory of images with --expectations (-r).

dm/DMUtil.cpp

 #include "DMUtil.h"
 
 #include "SkPicture.h"
 
 namespace DM {
 
 SkString UnderJoin(const char* a, const char* b) {
     SkString s;
     s.appendf("%s_%s", a, b);
     return s;
 }
 
-SkString Png(SkString s) {
-    s.appendf(".png");
-    return s;
-}
-
 void RecordPicture(skiagm::GM* gm, SkPicture* picture, uint32_t recordFlags) {
     const SkISize size = gm->getISize();
     SkCanvas* canvas = picture->beginRecording(size.width(), size.height(), recordFlags);
     canvas->concat(gm->getInitialTransform());
     gm->draw(canvas);
     canvas->flush();
     picture->endRecording();
 }
 
 void SetupBitmap(const SkBitmap::Config config, skiagm::GM* gm, SkBitmap* bitmap) {
@@ skipping 10 matching lines @@
     canvas.drawPicture(*picture);
     canvas.flush();
 }
 
 bool BitmapsEqual(const SkBitmap& a, const SkBitmap& b) {
     const SkAutoLockPixels lockA(a), lockB(b);
     return a.getSize() == b.getSize() && 0 == memcmp(a.getPixels(), b.getPixels(), b.getSize());
 }
 
 }  // namespace DM