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
9c398de683