Fixes#1722
Previously, SharedGrid.getIndex would properly lock any access to search
for and load the face metadata for a font face that contains a
codepoint. However, that face may be "deferred" (metadata loaded but the
actual face not loaded).
Later, outside of the SharedGrid write lock, a deferred face may be
initialized and cause a data race if two threads are doing this at the
same time, sometimes loading to a crash.
This commit fixes the issue by always preloading font indexes in
getIndex because the usage of getIndex implies a very near term future
use of getFace.
