Now if u_logging is used, it goes to logcat.

Uses Linux byteorder includes.

Things like ffmpeg aren't there but were found on the host.

This reverts commit 8c7aed82.

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.