Implement glyph width/xoff cache for QFontEngineS60::recalcAdvances(). Set advances_y[i] to 0.

Other ports cache the glyph widths which get read millions of times in QFontEngineXYZ::recalcAdvances(). Windows implements a lookup (ultrafast/memoryhungry), others a hash (fastenough/kindofmemoryefficient).
Let's benchmark how the cache affects text layout on Symbian always keeping in mind that at least one int per glyph per font variation gets reserved.

Set advances_y[i] to 0, since glyph_metrics_t.yoff is zero, also on all other platforms. We do not support vertical text flow (, yet).