ofPixels different bit depth support

[dimitre]

I think this one is important for 0.12.1
Once we discovered ofPixels was not good for 16bits per channel some things were fixed,
revealing other issues.
I suggest we review the code and update the tests accordingly

here is one current issue
#8225

and its fix that will break tests (we should update tests to reflect what is needed)
#8226

[dimitre]

closed by #8226