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`.
In order to save redundancy with initializing empty fields with NULL / 0
values and improve readability of the Vulkan code.
This patch also uses struct initializers where possible.
This essentially reverts 1eae45212e.