This adds an Init() which should be called before any resources
are loaded or GMs/Tests are called.
This also adds a little helper in compile.sh for building only
a single GM, which can make local development faster given the
fact that emsdk doesn't do its final compilation/linking in
parallel.
Bug: skia:10812
Change-Id: I1a8932549b9ae951c153c0d49927bdc933c29657
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/329358
Reviewed-by: Ben Wagner <bungeman@google.com>
acdc283404