Fix multi-GPU support for Nvidia

[PlasmaPower]

This PR has two commits to fix running aquamarine against multiple Nvidia GPUs which provide different output monitors:

• A small commit to not force linear allocation on devices which don't support it, even if multigpu is enabled. This is necessary as Nvidia doesn't support the linear format.
• A Renderer commit to, if creating an EGL image from the source buffer fails, instead copy it to an intermediate buffer on the CPU memory first, then copy that intermediate buffer to the output buffer. This fixes Nvidia multigpu because Nvidia doesn't support attaching another gpu's buffer as an EGL image, so instead this PR first reads the source buffer into CPU memory using the primary renderer's EGL context, and then does the rest of the blit as normal on the secondary GPU which writes to the output buffer.

[vaxerski]

I am a bit torn on the second one. That copy is very slow. Will it not lag into oblivion?

[PlasmaPower]

It manages 55 fps on a 4k display on my system which is not great, not terrible. Mutter does the same copy from what I can see and I'm not aware of an alternative.

[vaxerski]

It manages 55 fps on a 4k display on my system which is not great, not terrible. Mutter does the same copy from what I can see and I'm not aware of an alternative.

alright.