Add ability to draw seperate layers

[atomicptr]

e.g. only draw layer 2, 3 and 6 or only layers with specific names

related to #52

[johnhattan]

Making a layer's "visible" member in layer.hx write-able would let you turn off layers before re-rendering them.

public var visible(default, default):Bool;

Mind you, that's a pretty limited solution, but at least it's an easy one to implement :)

[atomicptr]

At the moment they're only rendered once so there is no point in making this writable again. I might want to add the ability to draw layers seperate from the map, but dunno.