Rename VRDisplayCapabilities#canPresent to be less ambiguous for detecting "magic window"

[cvan]

IMO, <VRDisplay>.capabilities.canPresent seems a bit odd (to be a property of the capabilities object and the canPresent naming).

perhaps instead have <VRDisplay>.isRealDisplay (à la OpenVR's IsDisplayRealDisplay)?

[toji]

@cvan knows this, but for the benefit of anyone else: The proposal for 2.0 here is that you attempt to create an exclusive (presenting) or non-exclusive (magic window) context, and the implementation can fail in either case. There's also a supportsSession function for testing if the given session type is supported without actually creating one. That should generally solve the issue of this being an awkward capability bit.

[cvan]

yeah, this seems good - thanks for the update