Using Vc::reciprocal() and SSE instead of division in OverCompositor32

Summary:
Instead of the usual division, we can use the division of 1 by the
number (reciprocal) and multiplying it by the dividend in the original
fraction. Also, the use of Vc::reciprocal() and intrinsics using SSE in
total gives us an acceleration of up to 30%.

Reviewers: dkazakov, Krita
Reviewed By: dkazakov, Krita
Subscribers: alvinhochun, woltherav
Tags: Krita

Differential Revision: https://phabricator.kde.org/D7314