Add padding value for mask

[puhoshville]

I have noticed that the image and ground truth are processing in the same way, but this entails difficulties when they have different color schemes. So I made a new do_mask function that handles ground_truth images.

Example of usage:

    p.rotate_without_crop(
        probability=0.7,
        max_left_rotation=10,
        max_right_rotation=10,
        fillcolor=(255, 255, 255),  # will apply to image
        mask_fillcolor=127,  # will apply to grayscale ground truth 
    )

Relates to #165

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This change is 

[mdbloice]

Hi @puhoshville I will take a look a closer look at this soon, and merge it once I've done so. Thanks for the efforts!
M.

[puhoshville]

Did some refactoring.
Also, there is an issue with white lines and I don't know how can I fix it:

[mdbloice]

Hi @puhoshville are the white lines something to do with aliasing maybe? And that anti-aliasing needs to be applied which may not be the case by default (I cannot remember off the top of my head).

[puhoshville]

@mdbloice sorry I didn't check it before - there is white lines problem on commit e6ef7bc - it's before my contributions, so I created a separate issue to handle it. You can review current PR and merge it if all is ok.

[puhoshville]

@mdbloice, any updates?