Move BitmapFunction out of the playground

[sdrave]

This should be an interesting Function for many users, so we should not hide it in the playground. Before moving, we should check again the implementation, though.

[sdrave]

@andreasbuhr, according to this documentation, an image mode 'L' should imply 8 bit grayscale, so the additional check in __init__ would not be necessary. I can also reproduce locally that an 16bit grayscale image has mode 'I'.

What version of PIL have you been using? According to this, there is supposed to be a depth attribute on the Image object which, however, is missing in my PIL version. If I remember correctly, you did not have it either?

[andreasbuhr]

I reproduced the problem I had. Indeed, it's sufficient to check for 'L' mode. The image I had problems with reports an 'I' mode.

[sdrave]

Done in 5f426dd.