Scanline decoding for bmp

Redesigns SkScanlineDecoder.h to indicate the ordering
in which the scanlines are provided

Refactors SkSwizzler::Fill() to include the zeroInit check
and to actually be correct.

BUG=skia:3257
BUG=skia:4198

Review URL: https://codereview.chromium.org/1287423002
22 files changed