Stop creating CodecSrcs with unused scale

Stop creating CodecSrcs with unused scale

Only JPEG and WEBP support native scaling, so only create a CodecSrc
with a scale for those types.

If I run

    dm --src image --images resources

this cuts down the number of Srcs from 11063 to 8032. All of these
trimmed Srcs would have failed quickly for each Sink (3 Sinks with the
above flags), but this will avoid creating them.

It drops the runtime on my mac from 13.2s to 11.4s, for about a 14%
speedup.

BUG=skia:5307
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1999103002

Review-Url: https://codereview.chromium.org/1999103002