glglyphcache: Fix dropping caches unnecessarily

glglyphcache: Fix dropping caches unnecessarily

The first set of glyphs is created with a timestamp of 1. Later we
subtract the glyph timestamp from the cache timestamp, meaning we end up
with numbers ending in 9, e.g. 59. Now unfortunately !(60 <= 59), so we
do not end up incrasing the old_pixels count of the cache. Later we then
call lookup() and DEcrease the old_pixels count, which makes the
unsigned int wrap and cause a huge old_pixels value, which causes us to
drop the cache.