[libkwineffects] Push render targets more efficiently in GLRenderTarget::pushRenderTargets

Summary:
If s_renderTargets is not empty, GLRenderTarget::pushRenderTargets starts
to do pretty heavy things: it deletes head of the targets param in a while loop.

There is no need to do that. Because QStack inherits QVector, we can use
append method to push new render targets in a more efficient way.

Test Plan: Background behind Konsole is still blurred.

Reviewers: KWin, mart

Reviewed By: KWin, mart

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D13823