Mechanically replace ExternalImage with EGLImage to better reflect their affiliation with EGLImage.
<rdar://problem/110814538>
Pull request: https://github.com/WebKit/WebKit/pull/14987
Committed 265232@main (b93c2f660b51): <https://commits.webkit.org/265232@main> Reviewed commits have been landed. Closing PR #14987 and removing active labels.