Fix missing glyphs when using shared distance-field cache

Fix missing glyphs when using shared distance-field cache

When using a shared distance field cache, signals such as
itemsMissing() and itemsUpdated() can be triggered in the
server process by external requests. This, in turn, can
lead to unnecessary storeGlyphs() calls (performance hit)
and, even worse, calls to setGlyphPositions() for glyphs
which have not previously been requested through populate().
The latter could cause missing glyphs when the same
characters were requested later, as they would then be
stored in the cache with the bounding rect of a default
constructed QPainterPath (in setGlyphPositions()).

To fix this, we ignore all glyphs which have no been requested
in this process, thus filtering out all events for glyphs which
have only been used externally so far. I've also added an
assert to the setGlyphPositions() to help us catch this case
early if it should return.

Change-Id: Ief333f612e4affea768d9c60409d43ce29fe3f60
Reviewed-by: Jiang Jiang <jiang.jiang@nokia.com>