We've had this struct for a while. It'd be useful for compositors
if they want to manage the swap chains themselves instead of being
forced to use wlr_output's. Some compositors might also want to use
a swapchain without an output.
The swapchain maximum capacity is set to 4, so that we have enough room
for:
- A buffer currently displayed on screen
- A buffer queued for display (e.g. to KMS)
- A pending buffer that'll be queued next commit
- An additional pending buffer in case we want to invalidate the
currently pending one
