Pass the destination pointer to next() in SkSwizzler

Per our discussion, we can make the swizzler simpler and more usable
for SkCodec and SkScanlineDecoder by only having a single version of
next() which takes a pointer to the srcRow and a pointer to the
dstRow.

BUG=skia:

Review URL: https://codereview.chromium.org/1256373002
8 files changed