Fix long standing bug where load* functions could not use callbacks in preload()

[Jared-Sprague]

Issue: #949

The problem was that the decrementPreload() function was getting pushed on the argument list for load functions in _wrapPreload() and when no success callback was passed to the load function decrementCallback() was assumed to be in the position of the success callback, but when a callback was passed in, decrementPreload would never get called.

[Jared-Sprague]

@lmccart This pull request includes unit tests and an example to demonstrate that it works

[lmccart]

hey @Jared-Sprague thanks for this! this definitely fixes the problem, but I feel a little unsure about repeating the same code lines in every function. is there a way to do this in just one place? either somewhere around here where preload is handled, or by making helper methods that get called in each loadX() function?

[Jared-Sprague]

@lmccart I don't think there is a way to fix this without modifying each load function, because decrementCallback has to be called inside the load function when in preload, it just used to be called in the place of the success callback. However I totally can extract the common lines from each load function into a helper method. I'll do that now!

[Jared-Sprague]

@lmccart Ok check my last commit diff and let me know what you think!

[lmccart]

if (typeof decrementPreload === 'function')

this looks great. one super nitpicky suggestion, would it make sense to put this above check inside the _getDecrementPreload() function, so that function either returns a pointer to the decrement callback function or null? then you could just check in the individual functions if (decrementPreload). does that seem right?

[Jared-Sprague]

@lmccart sounds fine to me! Stay tuned that will only take a minute.

[Jared-Sprague]

@lmccart Ok check it out. Note, I still have to check that (callback !== decrementPreload) because when no callback is passed in decrementPreload will be in the callback position and we don't want to call decrementPreload twice.

[lmccart]

great, thank you!

[Jared-Sprague]

@lmccart whoo hoo! Thank you too!