Ensure that SkColorTable->fCount is set properly after decodes

We now have some blits that will process the color table.

If we erroneously report that the size of the color table is 256,
we will do extra work and annoy MSAN.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1982753002

Review-Url: https://codereview.chromium.org/1982753002
3 files changed