Split SkBmpCodec into three separate classes

Will regress behavior on gold on test32bfv4.bmp, where we
will no longer fix transparent decodes.

TODO: Start fixing transparent decodes again, or decide
that we don't want to fix them and remove isTransparent
from SkSwizzler.  I think this may become more clear when I
start implementing the scanline decoder.

BUG=skia:

Review URL: https://codereview.chromium.org/1258863008
13 files changed