contrib: a shadertoy for shader expressions

You cannot see an expression by reading it. `(y % 2) * 40` is scanlines and
`(x > cols - 10) * 120` is a right margin, and neither is obvious until it is
on a screen -- so here is a screen. One HTML file, no build and no server:
open it and it works, offline, including from a checkout on a machine that
has never run sl0ppty.

It draws a fake pane with plausible contents, applies a chain of passes to
every cell, and prints the config line to paste. The mouse moves the cursor,
so `dist(x, y, curx, cury)` behaves the way it will in a pane rather than the
way it does at coordinates you imagined. Anything reading `t` animates. The
presets are every built-in written as an expression, which is the equivalence
the whole design rests on: `vignette` and `spotlight` are not special, they
are `dim` with an argument.

It also reports what each expression *reads* and whether that makes it
cacheable, because the dependency analysis is the interesting half of the
implementation and is otherwise invisible.

The hazard is that this is a second implementation of the language, in
another language, and it would disagree with the compiler quietly -- in the
third digit of a colour, on one preset, at one cursor position. So
tests/test_shadertoy.py lifts the evaluator straight out of the page (not a
copy of it, the page itself) and runs a few hundred expression/environment
pairs through both it and src/expr.c, including the corners: division by
zero, the ternary's untaken branch, `dist` counting a row double, and six
expressions that both sides must refuse. build/expr_eval is the real compiler
as a filter, which is what makes that comparison possible at all.

It found a bug on its first run, in the page: unary `-` and `!` did not
thread the environment through their closures, so `!x` threw instead of
evaluating. Which is the argument for the test, made better than I could have
made it.

Skipped with a message when node is missing -- a checker for a contrib toy is
not a reason the suite cannot run.

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