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Fix issue selecting fully transparent pixels in fill and contiguous selection tools

Deif Lou requested to merge deiflou/krita:deiflou/floodfill_sel_threshold_1 into master

Two fully transparent pixels can have different color components. This can happen when erasing with a brush (only the alpha component seems to be set). Those pixels should nevertheless be considered equal (transparent pixels).

In the difference policies used by the flood fill algorithm there is a special case for when the threshold is 1. In that case the pixels are just compared with memcmp and those fully transparent pixels with different color components are then considered not equal. A check for the fully transparent colors must be put in place similar to how differenceA does it, otherwise a contiguous transparent region may not be totally selected if the color components vary.

A video showcasing the issue (first half: old; second half: new): https://youtu.be/LMbiTx4DZrE

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Edited by Deif Lou

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