[libkwineffects] Push render targets more efficiently in GLRenderTarget::pushRenderTargets
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
Differential Revision: https://phabricator.kde.org/D13823