add newImage API

include/core/SkImage.h

Index: include/core/SkImage.h
diff --git a/include/core/SkImage.h b/include/core/SkImage.h
index 8d65870dc03883a2c8e1cfa954781ad8946d273d..6322afc978f104f6e206e6c92198383cc4b47c00 100644
--- a/include/core/SkImage.h
+++ b/include/core/SkImage.h
@@ -8,6 +8,7 @@
 #ifndef SkImage_DEFINED
 #define SkImage_DEFINED
 
+#include "SkFilterQuality.h"
 #include "SkImageInfo.h"
 #include "SkImageEncoder.h"
 #include "SkRefCnt.h"
@@ -132,6 +133,25 @@ public:
 
     const char* toString(SkString*) const;
 
+    /**
+     *  Return an image that is a rescale of this image (using newWidth, newHeight).
+     *
+     *  If subset is NULL, then the entire original image is used as the src for the scaling.
+     *  If subset is not NULL, then it specifies subset of src-pixels used for scaling. If
+     *  subset extends beyond the bounds of the original image, then NULL is returned.
+     *
+     *  Notes:
+     *  - it is legal for the returned image to be the same instance as the src image
+     *    (if the new dimensions == the src dimensions and subset is NULL or == src dimensions).
+     *
+     *  - it is legal for the "scaled" image to have changed its SkAlphaType from unpremul
+     *    to premul (as required by the impl). The image should draw (nearly) identically,
+     *    since during drawing we will "apply the alpha" to the pixels. Future optimizations
+     *    may take away this caveat, preserving unpremul.
+     */
+    SkImage* newImage(int newWidth, int newHeight, const SkIRect* subset = NULL,

[scroggo, 2015/01/22 20:05:14]

Why not take an SkImageInfo instead of a width/height? Right now we silently
give them kN32 Maybe that's okay, but we should perhaps document it? Unless we
think we might make a different decision later? That would also give them the
option of specifying the alpha type.

[reed1, 2015/01/22 21:52:52]

Being conservative. Since we don't even reveal the colortype or alphatype today,
it seemed premature to allow them to request something specific during resize.
If we hit some user that really needs more control, we can revisit the api then.
Is that ok?

+                      SkFilterQuality = kNone_SkFilterQuality) const;
+
 protected:
     SkImage(int width, int height) :
         fWidth(width),