Added a DestBufferAllocator callback to createDestImageData()

• Added a new DestBufferAllocator callback function and info pointer to createDestImageData().
	» By default, if the `destBufferAllocator` arg is null, uses an internal `defaultDestBufferAllocator()` function which just calls a `calloc()` as before.
	» The DestBufferAllocator is also expected (and enforce through a bool-union trick) to state via an out arg whether the CFData should take ownership of the allocated data (as before) or not (useful when re-using the same buffer for each draw).
• Updated ViewController to use a DestBufferAllocator (named `fetchDestImageBuffer_callback()`) to re-use the same draw buffer every time.
	» Also `memset()`s it to `0x00` (transparent black) when using OutsideOfQuadUVSkip mode (else we see previous-draw-ghosting artifacts).
• Minor: Removed old unused `createdByteCount` var from ViewController’s `- destImage`.
Minor: Removed lroundf() usage from genDestImagePixelBytes()

• Noticed while profiling that lroundf() was accounting for quite a bit of CPU.  Replaced it with a simple integer truncation (with a `- 1` correction for negative values).
• Also storing the float src width/height in DestImageGenInfo as a single GLKVector2 now, so we can easily use GLKVector2Multiply() in genDestImagePixelBytes().
Minor: Passing DestImageGenInfo as const-ref

• Now passing the DestImageGenInfo around inside of CGTextureMapping as a const-ref instead of const-pointer.
	» IIRC, this can be optimized by the compiler a little better, and is a little faster.  But it’s only possible in C++ though.
Fix: Changed imageView content mode to scaleAspectFit

• Changed the Main.storyboard imageView’s content mode to `scaleAspectFit` (the default) from `scaleToFill`— autorotations look better now.
Minor: Renamed createDestImageData() to cgTextureMappingBlit()

• Renamed `createDestImageData()` to `cgTextureMappingBlit()` (and all variants and uses), since the former name was specific to what ViewController was doing, while the latter describes the general functionality.