Fix glBlitFramebuffer for per-output rendering

Summary:
This is similar to ff6e042c7f9617004fefe63dd657fc2abf374c15 just for
GLRenderTarget instead of GLVertexBuffer. The reasoning is the same: on
Wayland KWin has one native window per screen and needs to adjust the
blit target depending on the area.

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D3056