Use ByteBuffer to convert from gray OpenCV Mat to PImage pixel array.

This changes only the single dimensional gray Mat input case. It also
assumes that the output PImage is in the ARGB (4 channel) format.
Use ByteBuffer to convert 4 channel color OpenCV Mat to PImage pixels.

Assumes that the output PImage is in the ARGB format.
Use ByteBuffer to convert 3 channel color OpenCV Mat to PImage pixels.

Assumes output PImage is ARGB format.
Merge pull request #64 from mudphone/to-pimage-with-buffers

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