Performance optimizations

Performance optimizations resulting from profiling in Instruments and tweaking accordingly.  Down to ~11.5ms per redraw (~85 FPS)!

• Fetching the data pointer out of the CFData and passing that via DestImageGenInfo, then just indexing into it in `genDestImagePixelBytes()`.
• `genDestImagePixelBytes()` optimized to always work with 4 components.
	» No longer returns the size; it’s always 4.
• Passing the pixelX and pixelY into `genDestImagePixelBytes()` rather than having it calculate it itself.
• Storing srcWidth and srcHeight in both int and float forms in DestImageGenInfo in order to avoid a cast.
• For now (in genDestImagePixelBytes()), just copying the `texelBytes` into the `pixelByteBuffer` instead of the intermediate `nearestTexelSample` step (which was intended for bilinear filtering).
• Probably some other minor things.

Other changes:
• Switched to bilinear coordinate calculation; currently, it’s much faster.
• Renamed genDestImagePixelBytesAtPosition() to genDestImagePixelBytes().
• Using a union in DestImageGenInfo so that it can be assigned all-at-once.
• LLVM Optimization Level for Release builds set from “Fastest, Smallest [-Os]” to “Fastest, Aggressive Optimization [-Ofast]”