Fix bmp bug in ReadHeader()

For BmpMaskCodecs, we need to skip any offset after the
header to get to the start of pixel data.

Before this fix, we only skipped this data when codecOut
is non-NULL (instead of on every call to ReadHeader()).

https://gold.skia.org/search?q=1&neg=true&unt=false&query=name%3Drgb16-565pal.bmp_0.200%26name%3Drgb16-565pal.bmp%26name%3Drgb16-565pal.bmp_0.250%26name%3Drgb16-565pal.bmp_0.333%26name%3Drgb16-565pal.bmp_0.375%26name%3Drgb16-565pal.bmp_0.400%26name%3Drgb16-565pal.bmp_0.500%26source_type%3Dimage
BUG=skia:

Review URL: https://codereview.chromium.org/1318393004
1 file changed