Vulkan is still required for most actual uses of Monado, but this
makes the coupling weaker.

Includes unit tests for oxr_input_transform, add to CI

This is a revised implementation using "variants" instead of
"inheritance" in a linked-list.

This silences some very large annoying warnings at configure time.

Instead of maintaining this chunk of code and build options, the Vulkan
loader can be used to load up validation. This has the advantage that no
layer name needs to be hard coded inside Monado, which was subject of
change recently.

Instead of using our own environment variable we can easily set the one
from the loader, e.g. `VK_INSTANCE_LAYERS=VK_LAYER_KHRONOS_validation`.

Also revise other small related parts of the build to make them more clear and consistent.

This enables out of process compositing.

Fixes #62.

Since there currently is no Vulkan extension that takes XCB handles to
aqcuire the display Xlib interop needs to be used.

Before this patch Monado was opening connections for both APIs, which
introduced overhead.

Even though all XCB handles can be casted to Xlib ones, this cannot be
done with the main xcb_connection_t / Display. In it's design the
interop between both APIs can create a xcb_connection_t from a XCB
handle, but not the other way round. So in an interop case the Xlib
connection is the main one, since it's on a higher level.
More information on this can be found here:
https://xcb.freedesktop.org/MixingCalls/

Unfortunately the clean solution for this would be to specify a Vulkan
extension that takes XCB handles. This would make sense since Vulkan
is aware of XCB in other parts of the API as well. In Mesa the Xlib
structs will be ultimately casted to XCB.

The existing thing would find pkg-config when cross-building for 32-bit,
but then immediately complain it wasn't found.

With loads of changes from Ryan.