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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

Now all the unittests in folders ./image/ ./ui/ and ./plugins/ pass
on my machine. The only exception is krita-fileformat-kis_exr_test, but
it probably related to the fact that I don't have all the needed libraries
installed.

Good news: now KisSelectionManagerTest and KisZoomAndPanTest do not take
30+ min to execute. The delay was related to the overfilled Qt event queue,
and now they take 90 and 40 sec correspondingly.

CCMAIL:kimageshop@kde.org

All the known bugs are fixed.

There is still one bug in both the composites: the calculation
of a single pixel compositions should be done in float instead of
integers, otherwise it causes artifacts on the canvas during painting.

It gives 1.58...1.74 times better result of the composition
on Sandy Bridge. Other architectures are to be tested.