Enhance eGPU support.

[Mikilio]

I would like to keep this issue open as a tracker for improving the experience with eGPU.
This only concerns systems satisfying all requirements:

• integrated GPU connected to the motherboards PCI bus
• A motherboard or Chipset supporting Thunderbolt
• A hot plugged GPU connected via Thunderbolt

Use cases

Dedicated GPU mode

Use iGPU as primary DRM and use eGPU as dGPU

External only

Use eGPU as primary DRM when available at start.

Parallel mode

Start two DRM backends, each managing only the outputs directly connected to it.

Mode	No Changes Needed	Good Performance	Supports Hot Plug
Dedicated GPU	✅	❌	✅
External Only	✅	✅	❌
Parallel Mode	❌	✅	✅

Possible Enhancements

DRM handoff

Allow for a way to manually change the primary GPU on the fly. This would allow the "External only" setup to be hot pluggable.

Implement Parallel Mode

I would need some feedback on how feasible this is

TODO's

to be added

[Mikilio]

I would love to use an eGPU and adding one of the two or both enhancements would be a big step for wayland desktops as a whole. I was hoping to receive some feedback on the feasibility. I checked the code to see what work has been done on that end, but I apologize if I still have some misunderstandings about it.