Subtract start_coord instead of adding it

Subtract start_coord instead of adding it

crrev.com/2420843003 (DIFFERENT ISSUE) introduced some changes in
sampled images. (I already corrected the problem for interlaced PNGs
in crrev.com/2424353003.)

When deciding whether a row is needed, we need to subtract the starting
coordinate, similar to how we subtracted fFirstRow in SkPngCodec.

This should "fix" the remaining untriaged images in Gold (i.e. we will
go back to producing the original image).
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2440563002

Review-Url: https://chromiumcodereview.appspot.com/2440563002