Check glyph size before adding to glyph texture cache.

It was noticed that we do not check if a glyph will fit in the hardware texture cache before trying to add it, even though the information is readily available. Instead we leave it to the texture code to check it, and then catch the exception it throws and then return.
This might all be OK, since the net result is the same but we also print the stack trace of the exception. If this occurs it would be unwelcome noise. That should be wrapped in a check to see if PrismSettings.debug is set. And at the same time do the check up front, so it is less likely to happen anyway.