Change image format for tests

[flrs]

The currently used "PNG" format is not lossless and therefore might influence the quality of the tests. For example, if a prepared test image is exported from GIMP for comparison with blend_modes' image blending method, the image might already be changed during the export process.

To avoid this problem, a lossless format should be used, for example TIFF. The format needs to be fully compatible with the library used for importing images into Python (currently opencv-python).

[MadsDyrmann]

I think you confuse the two formats. PNG is always lossless, whereas TIFF can be lossless, but often isn't. -- It depends on which compression method you select when exporting the TIFF image.

[flrs]

Alright, thank you for the explanation. Whatever we make of this issue, we should enforce a lossless format.