shadowblur: Speed optimization and major clean-up.
shadowblur: Speed optimization and major clean-up.

Implement vertical blur because horizontal blur with transformed image
is not too efficient since we only need the transformed alpha channels.
Handling of pathological case, where the blur radius is very large, is
now much faster. Unify the successive box blur operations in a loop.
Unroll the initialization of the sliding algorithm. Use direct pixel
access instead of QImage::scanLine. Fix the accumulation error by
rounding the value properly.

ariya/X2,ariya/X2,ariya/X2