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.

[ikalco]

this should also fix #134 by using the copy path in your second point

[PlasmaPower]

I don't have the hardware to test it but I don't think it'll fix that one actually. In my case (two Nvidia GPUs) it was failing to create the from buffer which triggers the CPU copy fallback in this PR, but in the linked issue (an iGPU as primary + an Nvidia dGPU as secondary it sounds like?) it looks like it was failing to call glEGLImageTargetRenderbufferStorageOES on the to buffer from the logs. I'm not sure which Renderer blit is being called on in that case and which GPUs the buffers belong to, so I'm not sure what the correct fix for that issue would be, but it would probably build on the infrastructure in this PR.

[PlasmaPower]

Fixed the merge conflict with #150

[vaxerski]

thanks!