This is a largely boilerplate type, so that the swapchain we're going to
add isn't tied to a specific allocator.

This type only very loosely wraps over allocators, only exposing a
alloc/dealloc function. Renderers using specific allocators are expected
to use those types directly.

This types gives a queue of wlr_images that are suitable to be presented
to the backend.

This is an instance of wlr_allocator for hardware rendering via OpenGL
or Vulkan. This also adds the appropriate interfaces to wlr_backend for
this to work.

We use minor numbers to detect if backend's renderers are compatible.
It also doesn't make sense to call attach_gbm or detach_gbm on a
multi-backend, as it has no idea which subbackend its supposed to pass
it to. Instead wlr_output.backend should be used.

This types adds a container for formats + modifiers.

A list that is of [format [modifier]] was chosen instead of
[format modifer] because that is how GBM accepts them.

This has a _2 suffix to not conflict with the current API, which can be
removed later.

This fixes the build on FreeBSD, which does not have these.

This just contains a couple of utility functions for interacting with
wlroots' types.