ofxiOSVideoPlayer API updates

- replaced getPixelsRef() with getPixels()
- replaced getTexture() with getTexturePtr()
- introduced ofPixels and removed old `unsigned char` pixel arrays.
- replaced pixel order swizzling functions with vImage methods from Accelerate framework (same as in the OSX version of AV video player)
- updated ofxiOSVideoPlayer example with the API changes.
- added Accelerate.framework to iOS template project.
const is not so strict that you can't modify anything, it's just a hint that says that the behaviour of this class is not going to change if you call that method, and allows to call it when you have a const reference to it. for example you can have a method that gets a const ofHasPixels like:

```
ofPixels threshold(const ofHasPixels & p);
```

if you dont't have the

```
const ofPixels & getPixels() const
```

threshold can't get a const renference to call getPixels on it and makes the api more confusing because now you don't know if threshold modifies the original pixels or not.

anyway since what you are modifying is only modified because of some kind of lazy initialization it's ok to modify it from the const getPixels since that won't change the apparent behaviour of the class. just do:

```
ofxiOSVideoPlayer* mutThis = const_cast<ofxiOSVideoPlayer>(this);
```
and use `mutThis->` to access anything that you need to mutate

you can also mark the pixels as mutable in the declaration like:

```
mutable ofPixels pixels;
```

but that's usually better to use only for flags and variables that you don't care that they are mutated anywhere in the class, in this case pixels shouldn't be changed anywhere else but in getPixels where it's lazily initialized if needed