add runtime option to provide data->imagegenerator factory

include/core/SkGraphics.h

Index: include/core/SkGraphics.h
diff --git a/include/core/SkGraphics.h b/include/core/SkGraphics.h
index ea74c586d2d8f92bef3a4b0d8544e878db1ae9ad..e552633d5f3df3aa32c8532f1aaf1bef8ef4d259 100644
--- a/include/core/SkGraphics.h
+++ b/include/core/SkGraphics.h
@@ -10,6 +10,9 @@
 
 #include "SkTypes.h"
 
+class SkData;
+class SkImageGenerator;
+
 class SK_API SkGraphics {
 public:
     /**
@@ -140,6 +143,16 @@ public:
      *  global font cache.
      */
     static void SetTLSFontCacheLimit(size_t bytes);
+
+    typedef SkImageGenerator* (*ImageGeneratorFromEncodedFactory)(SkData*);
+
+    /**
+     *  To instantiate images from encoded data, first looks at this runtime function-ptr. If it
+     *  exists, it is called to create an SkImageGenerator from SkData. If there is no function-ptr
+     *  or there is, but it returns NULL, then skia will call its internal default implementation.
+     */
+    static ImageGeneratorFromEncodedFactory GetImageGeneratorFromEncodedFactory();

[scroggo, 2015/07/09 19:01:15]

I do not understand the need for this accessor. Why not just call
NewFromEncoded?

(Ah, it appears the the test uses it to set it back. Is this useful outside of
tests? Even inside the tests, it seems less than ideal, as other tests might
change or use it, as you acknowledge when you mention it's racy.)

[mtklein, 2015/07/09 19:04:57]

I guess we could merge the two as

static ImagGeneratorFromEncodedFactory
SetImageGeneratorFromEncodedFactory(ImageGeneratorFromEncodedFactory), where the
first ImageGeneratorFromEncodedFactory is the previous value.  (Yes I typed all
that out.)

[reed1, 2015/07/09 19:05:45]

Not sure what to do here. Having a global is just a pain no matter how you spin
it.

+    static void SetImageGeneratorFromEncodedFactory(ImageGeneratorFromEncodedFactory);
 };
 
 class SkAutoGraphics {