Noticed by Joachim Mairböck after I hastily merged change 7402.
Also add named variation instance selection for fonts loaded from memory
areas like there already is for files.
Change-Id: I76de22181dc39559956c25987165be2f8a8c4a37
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7493
Tested-by: Commit checker robot <[email protected]>
Reviewed-by: Jérôme Duval <[email protected]>