Sometimes it is convenient to pass the result vector with more than a 3
elements, e.g. when passing QVector(4) with RGBA values. In this case
old implementation resized vector to 3 values.

This patch fixes assertion failure in KoColorSpace::decreaseLuminosity:
out-of-bounds access to channelValues vector

It occurs because we store the number of channels before
linearize/delinearize and then iterate over the channelValues assuming
that its size was not changed.

With this patch, Alpha channel is ignored during linearization and
vector size does not change.

Patch by Andrew Savonichev <andrew.savonichev@gmail.com>(asavonic) Thanks!