Fix both imageviewers on X.org with DefaultDepth 30

[pjanx]

Fixes #51.

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[pjanx]

@googlebot I signed it!

[nigeltao]

Thanks!

I have some minor quibbles, but I'll fix them in follow-up commits.

[nigeltao]

Having example/imageviewer use xrender means that it was now straightforward to add a zoom/magnify feature.

I also made some other tweaks. Let me know if I broke anything on DefaultDepth 30.

[pjanx]

I totally missed the build-example.sh script, or forgot about it at some point.

Indeed. Good gardening work, it's still working. You may simulate the situation by setting RGB24 on the application's main window, or the root window (which are both particularly annoying changes to make).