Adds a new material choice for a physically based metal/roughness
material. Options are available for setting the material base color,
metalness and roughness.
Internally this uses a clone of Qt's QMetalRoughMaterial class. We
use a copy of the Qt class instead of relying on Qt's implementation
as longer-term improvements (such as data defined base color) will
require a re-implementation anyway. By using our own material we
will avoid having two different code paths for the data-defined/
non-data defined scenarios.
