Enable blending if a subsurface has an alpha channel

Summary:
While investigating BUG 387313 I noticed that blending might be disabled
for subsurfaces. Blending was disabled before rendering the subsurfaces
and it is not checked whether the surfaces have an alpha channel or not.

This change addresses this problem by disabling blending after all
subsurfaces have been rendered and enabling blending if a subsurface has
an alpha channel.

Unfortunately this does not fix the investigated bug.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10060