Allows PathWalker to be used without creating a vtable

[ginnyTheCat]

This removes the creation of a vtable optional for PathWalker by making the Path::walk method generic over PathWalker. As PathWalker is implemented for all &mut PathWalker passing &mut PathWalker or &mut dyn PathWalker to the walk method is still possible.

It also adds the optional curve_to_y and rect methods to the trait that default to calling the methods that mupdf would call as well if the function pointers in the fz_path_walker struct were None. This can sadly not be done for curve_to_v and quad_to as both of them require knowledge of the start position to call the default, which is not available to as as the trait.

This also changes the parameter type of the Path::rect function from i32 to f32 as this is what the C API seems to pass to us.