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Closes #49

Closes #57

The variable should be set to the index in the enumeration of a modes according to VK_KHR_display.

Monado can print a list of available modes with their indices with the env var XRT_COMPOSITOR_PRINT_MODES=1.

To not clutter the user's output with debug info, add a variable to print only the available modes.

This value is used in xrWaitFrame() to throttle the application's rendering
down to the refresh rate of the HMD.

Best mode means: first maximize the rendered pixels, then maximize the refresh rate.

OpenHMD can't easily tell us the WVR SKU because it has to read it
from the config after opening the device, while the product string
is set at device list time.

As a workaround, recognize the WVR2 by its resolution.

Ubuntu 18.04 is the only debian based distro currently that doesn't have
cmake 3.13 packaged. And Ubuntu 18.04 is not supported for other reasons.

Closes #50

By request of @pblack.