Add missing client swapchain synchronization
Monado was failing to synchronize GPU access to OpenXR swapchain images between the client and the compositor, producing undefined behavior.
I'm unable to test this because some time between e7e550fa and current master, monado stopped displaying any graphics when used with my application. This may be related to regressions of the layered swapchain support I previously fixed in !42 (merged).
This does make the validation layers unhappy but I suspect that's a false positive. Confirmation TODO. It's a false positive that will be fixed in the next SDK: https://github.com/KhronosGroup/Vulkan-ValidationLayers/issues/1438