Add an interface for decoding scanlines, and implement that interface
in the PNG decoder.
Use a separate method to determine whether an image that used a type
with alpha was actually opaque.
SkScanlineDecoder.h:
New interface for decoding scanlines.
SkCodec.h:
Add getScanlineDecoder.
Add a virtual function (with non-virtual caller) for determining
whether the image truly had alpha. The client can call this to
determine if the image was actually opaque if it reported having alpha.
Remove code to sneakily change the passed in alpha type.
SkCodec_libpng.*:
Split up code onGetPixels into helper functions that can be shared with
the scanline decoder.
Implement scanline decoding.
Implement onReallyHasAlpha.
SkSwizzler.*:
Add a new SrcConfig as a default, which is invalid.
Add a function for setting fDstRow directly.
Assert fDstRow is not NULL.
BUG=skia:3257
Review URL: https://codereview.chromium.org/1010903003
