[effects/blur] Check for blitting support

Summary:
In D12678 blur was changed to use blitFromFramebuffer() instead of glCopyTexSubImage2D()
Now it checks if the GPU supports it.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D13246