SetPixel for 16bpp

[GoogleCodeExporter]

This is not a defect but an enhancement request.

While using the library I realized that a function to set 16bpp grayscale 
pixels is not available and the existing SetPixel() functions in 
ImageLineHelper.cs are based on an RGB model. So it doesn't work to use these 
with the same values for r,g, and b.

So I wrote my own and would kindly ask to add this to the code stack after a 
review by the gurus of this library.

My code is:

public static void SetPixel16bppGray(ImageLine line, int col, int gray)
{
    int offset = col * line.channels;
    if (line.IsInt())
    {
        line.Scanline[offset] = gray;
    }
    else
    {
        line.ScanlineB[offset++] = (byte)gray;
        line.ScanlineB[offset] = (byte)(gray >> 8);
    }
}

I hope this contribution is valuable.
Keep up the good work!

kr
Michael

Original issue reported on code.google.com by MichaelD...@gmail.com on 5 May 2015 at 1:24