Replace some fixtures with provably free content (#46)

[ferdnyc]

It seems that, as @aviau noted in #46, that at least the sample.tif file in the fixtures directory is non-free. The file contained a message right in the image: "This file is distributed with Techsoft PixEdit as a sample file, and is used with permission from the document owner." (That permission would NOT, in most countries' copyright law, implicitly extend to anyone OTHER than Techsoft.)

Some of the other files in the directory appeared similarly suspect, or at least there was nothing to indicate that they ARE free content. And since there's an absolute wealth of free content out there, for at least certain formats, it just makes sense to use anything other than free content. So, this PR gets the ball rolling by replacing three "low-hanging fruit", including and especially the Techsoft TIF image.

• sample.gif is a CC-BY-SA licensed image via Wikimedia Commons
• sample.tif is a public-domain Hubble Space Telescope image (thanks NASA!)
• sample.webm is a CC-BY licensed video via Wikimedia Commons

This change does not come without some tradeoffs in terms of file size.

• sample.gif grows from 3.3 KB to 390 KB, but it's a far better test file for it
• sample.webm grows from ~ 230 KB to ~ 330 KB, fairly minor
• sample.tif grows from 209 KB to an obscene 5 MB, and I do apologize for that, but one of the claims is that the tests are run on "real files", and it is hard to find SMAL "real" TIFF images. There is a strong bias for prioritizing quality and resolution over compactness when storing data in TIFF form. (I had to search through quite a few NASA galleries to find a file that small — others were tens or many hundreds of megabytes, some over a full gig!)

I personally think the increased sizes are a reasonable tradeoff, and 5 MB in today's terms is really not that big a deal. But if it's unacceptably large, I can keep looking for a smaller replacement for at least the TIF file.

Last but not least, a new file fixtures/sources.txt provides provenance details for all three files, including the applicable free content license details and any required attributions.

Partly addresses: #46

[h2non]

Thanks for that, boring stuff but quite useful to keep things healthy in terms of free licensing.

The only point is about having a smaller TIFF image would be better. Up to 1MB seems reasonable to me.

[ferdnyc]

@h2non Agreed, I'll keep an eye out for a good source for a smaller TIFF sample, probably outside of NASA since they do love their pixels.

And I'll try to "libre-up" "libre-ate" (duh) some more of the existing sample types, where I can find replacements. (I have to think either the US government or Microsoft themselves must have files in standard office document and/or archive formats that they've cleared for reuse and redistribution.)