Don't 'require' external image extensions, 'enable' them

We trigger this when we don't necessarily have support for both
extensions, so requiring them can cause compilation failures.

Bug: https://github.com/flutter/flutter/issues/16178
Change-Id: I54c190709a677bc9ec2fa46f612e8958e3de38cb
Reviewed-on: https://skia-review.googlesource.com/123683
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed