[libkwineffects] Set original backend framebuffer for render targets

Summary:
KWin only renders into the default framebuffer, which is for example an EGL
surface.

To prepare a post-processing step with a different framebuffer allow the
framebuffer to be changable. For that KWin's current framebuffer must be
communicated to the GLRenderTarget class, which otherwise does not set it back
to KWin's current one when a render target is disabled again.

Test Plan: Compiles, with other patches for Gl based screen rotation

Reviewers: KWin

Subscribers: fredrik, zzag, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D25904