Totally double-protect Layer class

[euzuro]

See #636 for more info, but the basic idea is that sometimes a layer will be a subclass of two different parent layers. then, at each step of the game, the correct way to deal with bubbling up calls to the superclass is to bubble them up to *both* superclasses. Unfortunately, as this plays itself out, that means that stuff here in layer will be getting called twice. No reason to put ourselves out so.

I realize this isn't the most elegant thing, but it's the only way I see proper to do it.

[euzuro]

for each function that might get double-called due to split subclassing, make a test condition to check if it's already been done before. most important of these is in the initialize() function.

[tschaub]

can't initialize just have one condition: if(this.id == null) {} ?

[euzuro]

updated changes to constructor to follow tschaub's advice. makes it a bit more readable. thx!

[crschmidt]

Please commit, and mark pullup.

[euzuro]

r3035

[crschmidt]

Brought up to 2.4 branch for RC2 with r3088