ttf-driver: optimize TrueType_Gen_Widths

[jirkakunze]

Ideas:

1. Refactor filling CTF_IS_FIRST_KERN and CTF_IS_SECONT_KERN in ConvertWidths().
   At the moment we iterate over all char and for each char over all kernpairs. This results in hundreds of thousands of iterations to fill the chartable entries.

Better: Iterate over kernpairs and fill for every kernpair CTF_IS_FIRST_KERN and CTF_IS_SECONT_KERN flag. For this we only need as many iterations as there are kernpairs.

2. Write a function that loads the fontbox for the GEOS characters without loading the glyph. A similar function already exists in the ftxwidth.c extension.

[jirkakunze]

Here is the timer profile of the TrueTypeGenWidths function:

We need a better solution to convert the index of a character into a GEOS character.