Make ImageProvider.loadImage() abstract

[tvolkert]

In #118966, we added ImageProvider.loadImage(). This method should be abstract in ImageProvider, but due to the need for backwards compatibility, it was made as a non-abstract method to allow subclasses time to migrate.

Once the deprecation period for the old method (loadBuffer()) is over and that method is removed, the base ImageProvider.loadImage() should be made abstract.

[shogohida]

Hi @tvolkert, can I work on this? When will be the deprecation period for the old method (loadBuffer()) over?

[tvolkert]

Hi @shogohida, loadBuffer() was just just deprecated in #118966, so we won't be able to remove it (and make loadImage() abstract) for about a year.

[tvolkert]

This is ready to be done now, as we're outside of the deprecation period.