Make cache of opentype tables in Harfbuzz face lazy

Make cache of opentype tables in Harfbuzz face lazy

The mechanism in fontconfig which determines if a certain character
is available (FcCharSetHasChar()) may give false positives, in which
case we would load and unload those fonts per every char for which
FC gave us a false positive. This was a major performance regression.
Specifically the false positives happened when looking at e.g.
italic variants of certain multilingual fonts, since we only check
the charset of the font family as a whole and not of the specific variant,
which may only support a subset of the chars.

To optimize this, we remove the deletion of the font engines after
loading them, but also wait with loading the opentype tables until
they are actually needed. This means that for the false positives,
we will load the font, but the cached data for each unused font will
be much smaller.

Change-Id: Idfc794401a2080da5946bf65204eb947aeb635ed
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>