Use ByteBuffer to convert from OpenCV Mat to PImage pixels array.

I have noticed quite a good performance boost when I remove the for-loop in OpenCV#toPImage(). The use of ByteBuffers allows for bulk copying of the pixel data, but requires "ordering" of the bytes before converting to an int array (when channels > 1). One assumption that I've made is that the output PImage is in the ARGB format. I'm not sure if that is valid.

I am a n00b with Processing PImage pixel data, so I'm not sure I have been able to comprehensively test these changes. I did my best and have put some test sketches here: https://github.com/mudphone/opencv-processing/tree/to-pimage-with-buffers-test-sketches/examples/TESTS

There is a sketch for the single (gray), 3 (RGB), and 4 (RGBA) channel cases. I noticed a definite FPS increase when using this technique on machines with lower-powered CPUs (choppy FPS to 60 FPS). Even on a newer MBPro, I was able to see some improvement in the video sketches (50 FPS to >60 FPS).

Please let me know if there are any other cases you'd like me to test. Or, if I'm just plain missing some important use cases. I have only really used this for a single production sketch.

Credit for the idea to use ByteBuffer comes from Bryan Chung's blog: http://www.magicandlove.com/blog/2014/02/27/conversion-between-processing-pimage-and-opencv-cvmat/