Data transfer tweaks

• Switched createDestImageData() to return a ref-counted `CFDataRef` instead of a raw data pointer and size.
• Switched out `CGPoint` and `CGFloat` usage for `GLKVector2` and `float`.
	» CGPoint and its CGFloat can be a float or double depending on the CPU type; GLKVector2 always uses floats— so this avoids unnecessary conversion overhead.
• Changed `roundf` in genDestImagePixelBytes() to `lroundf`— a specialized round function specifically for taking a float and returning an int-type.
• Updated usage of createDestImageData() accordingly.