Allocate the struct 'tilda_window' on the stack of 'main()'.

The lifetime of the 'tilda_window' instance is exactly the same
as the 'main()' function (it doesn't get (de-)allocated anywhere
else), so we might as well allocate it on the stack of the 'main()'
function.

Apart from the (minor) benefit of avoiding a heap allocation,
this removes a bit of the necessary error handling.