Optimized vector composite ops by 1.5-2 times more

Conversion Uint<->Float is quite expensive in comparison to
Int<->Float (2-2.5 times). This happens because of special code
that handles sign bit of the number. So discarding this bit with
conversion Uint->Int makes a huge speedup.

Now the vector version of the composition is 1.8-8.7 times faster
that the old version (weighted: 3.2 times).

Many thanks to Matthias Kretz for pointing this out!

CCMAIL:kimageshop@kde.org
CCMAIL:kretz@kde.org