[effects/blur] Update blur to be more natural

Summary:
This gets rid of the dark area that may appear between very different colors by doing the blur in SRGB colorspace.
This is not enabled for GLES, and will use the previous blur type.

Test Plan:
Before:

After:

Reviewers: VDG, KWin, davidedmundson, zzag, fredrik, ngraham

Reviewed By: VDG, fredrik, ngraham

Subscribers: Codezela, fredrik, abetts, Petross404, rapiteanu, filipf, rooty, ngraham, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D18377