add runtime registration for encoders

If we want to make these external dependencies mix-and-match in Google3,
we'll need to group together the encoders and decoders, everything that
dependends on each external library.

I was tempted to try to remove these generic encoder entrypoints and
replace them with calls to the direct equivalents, but I'm not sure
that's necessary.  I think we can just register encoders like decoders.

Change-Id: I41d2d1bb3ceb1daafa62c95d345eb6a70249be75
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/213880
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
6 files changed