Added test images

[radarhere]

Suggestions for python-pillow#7603

1. You will see in some of the other DDS tests we compare the opened image to a saved PNG version. So this adds that.

However, my macOS Preview application doesn't think that the DDS image looks like the PNG that Pillow created and I've included here - but then again, macOS Preview could be wrong.

So if you think the PNG I've included here is correct, then feel free to merge this.
Otherwise, let me know, as this is a problem to be investigated.

2. I've hexeditted your image to add a BC4 TYPELESS test image.

[sambvfx]

Thanks @radarhere!

The original PNG was simply the existing ati1.png from the tests.

You will see in some of the other DDS tests we compare the opened image to a saved PNG version. So this adds that.

I ran into issues comparing the original png to the dds version when it compared tobytes(). I'll check again to see if this test is passing.

I've hexeditted your image to add a BC4 TYPELESS test image.

Thank you!

[radarhere]

The compression is lossy format, so it is not unexpected that converting to it created a slightly different file.

Testing, I find that assert_image_similar_tofile(im, TEST_FILE_ATI1.replace(".dds", ".png"), 0.4) passes, but this assert_image_equal_tofile is a stronger test, so the current version is fine to my way of thinking.