contrib: the shadertoy presets, usable in sl0ppty

They only existed in a browser, which is a strange place for a terminal
multiplexer's shaders to live. Now each one is a file you can paste.

contrib/shaders/ holds all thirty-two, one per file, *generated* from the
presets in shadertoy.html by contrib/gen-shaders. The page stays the source:
it is where they are written, previewed and conformance-tested against the
compiler, and a hand-kept second copy would drift the day after it was
written. `gen-shaders --check` says whether the files are current, which is a
test, so the drift is a failing build rather than a stale example.

contrib/shader-tour cycles a session through them, in the shape theme-tour
already established — write the config the session watches and let the
watcher do the rest. It takes one shader, one group, or nothing for all of
them, and it resolves a short name (`torch`) as well as the whole slug
(`guides-torch`), because nobody remembers which group a shader is in. The
tour writes the theme alongside the shader: shaders resolve default-coloured
text through theme's default_fg/default_bg, so a preset judged without one is
being judged against a guess.

The new test is the one that matters. Every shipped file is loaded into a
real session and has to *change the cells of a pane* -- an example that
previews beautifully and does nothing when pasted is worse than no example,
and neither the toy's conformance test nor a config parse would notice. Three
of them are then checked for the specific thing they claim: scanlines
alternate row by row, indent guides land on every fourth column, a flat dim
is flat. One session, reloaded per preset, which is both quick and a fair
exercise of the path anyone will actually use.

Renamed the "the built-ins, as expressions" group to drop the qualifier from
its filenames; it was generating built-ins-as-expressions-scanlines.kdl,
which is a sentence rather than a name.

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