Improve frame rate on monitors attached to secondary GPUs in copy mode

This avoids expensive EGLImage recreation every frame as suggested by daenzer in #3461 (comment 2218928). This makes it possible to hit 144 FPS on a 144 Hz monitor attached directly to the NVIDIA dGPU in my test setup. The visual desktop smoothness doesn't match the reported FPS yet for me, but fullscreen games running at 144 FPS do seems to have the expected smoothness now, so a good improvement regardless (see also #3461 (comment 2221875)).

Also applies to other dual-GPU combinations that hit this copy path, such as with the nouveau driver with an NVIDIA dGPU, but those paths might be more efficient, or not; I haven't tested if these were problematic but the EGLImage creation was the most expensive part of the blitting on my system with an AMD iGPU (Mesa) and an NVIDIA dGPU (NVIDIA driver and open kernel modules), and this makes that go away.