Fix #5570: Set prototype.constructor for all derived classes

See #5570 for rationale.

I first fixed a couple of instances of ``.prototype = new THREE.Loader();`` in loaders which executed the constructor logic when setting up the prototype (but shouldn't).

Then I started adding all the ``prototype.constructor = ...`` by hand but ended up building a script to automate the task instead because... there are many classes :D. You can check out the script here: https://gist.github.com/elisee/ebe992cbfe4c5aac8a5f

I looked over the changes made by the script and believe they are all correct. I rebuilt the lib and checked a bunch of samples at random and compared with the online versions and they all seemed to run properly.