Make it possible to check what features have been enabled

[n0F4x]

With the help of the recent API PhysicalDevice::enable_extension_features_if_present we get a way to enable optional device extension features, but there is no way to ask what features have been enabled afterward.

The same getter is already present for simple device extensions and features.

I presume that adding this new function (are_extension_features_enabled) also becomes easier with the addition of enable_extension_features_if_present.