Initialize offset memory for PyImagingPhotoPut

[nqbit]

When using image mode "1" or "L" block.offset[3] is never initialized. In some use cases the block is passed to Tk_PhotoPutBlock (https://github.com/tcltk/tk/blob/master/generic/tkImgPhoto.c#L2752) and block.offset[3] is stored in alphaoffset which is later referenced.

This change initializes block.offset[3] to 0 when using mode "1" or "L".

[radarhere]

Are you aware of a situation that shows this bug? We would be interested in adding such a scenario as a test.

[nqbit]

This was caught with a memory sanitizer, see the following example:

import numpy as np
from PIL import Image
from PIL import ImageTk

example_image_array = np.full((100,100), 123, dtype=np.uint8)
example_image = Image.fromarray(example_image_array)
example_image_tk = ImageTk.PhotoImage(image=example_image)

Is there currently any support for memory sanitization in the test framework?

[radarhere]

The closest we have to memory sanitization testing is #3961.

As such, while I agree that your change makes logical sense, I'm having trouble running a sanitizer over the code, since Tkinter requires a display, and my usual approach to this problem would be display-less docker.

[nqbit]

Thank you for helping to get this merged. One option for display-less configuration is xvfb (X virtual frame-buffer). https://en.wikipedia.org/wiki/Xvfb