Fix for running on x64

[genejo]

Thanks for the neat library!

Tried running after compiling for x64 and it broke. Found a fix - you may want to replace line 320:

var dst = new IntPtr(bmp_data.Scan0.ToInt32() + (stride * y));

with

var dst = IntPtr.Add(bmp_data.Scan0, stride * y);

to avoid manually interfering with pointer arithmetics.

[secile]

Thank you for your information.
In my PC, your problem is not occur, but I understand what you say.
Under x64, IntPtr value can be larger than 4byte,
therefore Scan0.ToInt32() can be cause overflow exception.

I'll fix this problem future release. Thank you.