Rely on wlr_output's generic swapchain support instead of creating our
own. The headless output now simply keeps a reference to the front buffer
and does nothing else.
Instead of managing our own renderer and allocator, let the common
code do it.
Because wlr_headless_backend_create_with_renderer needs to re-use
the parent renderer, we have to hand-roll some of the renderer
initialization.
When the headless backend uses an already-existing renderer, it doesn't
have ownership over the renderer. When the renderer is destroyed, the
headless backend needs to destroy itself.
This improves the way the output numbers are handled for the headless
backend. Instead of using the number of active outputs plus one, the
last used number is stored and new outputs will increment it. This
fixes the situation where you start with one output, create a second,
close the first, and create a third. Without this, both outputs will be
HEADLESS-2, which causes an issue since the identifier will also be
identical. With this, the last output is HEADLESS-3 and the outputs can
be distinguished.
This backports some changes to #319 to fix the screenshooter data
format. This also adds wlr_backend_get_renderer which will be
useful to support multiple renderers.
