Fix the flat drop on Linux and morph its play/pause in one shape

The kick wave in the vertex shader squared a negative number with pow(), which
GLSL leaves undefined. Mesa returns NaN there, so on Linux the drop's position
pinned to the clamp: it stayed a plain round sphere without wobble, and its
normals split it into a dark yin-yang. The squares are written out now, and the
normal comes from the slope of the displacement instead of two whole positions.

The play/pause icon is one shape whose corners move: each pause bar turns into
half of the play triangle, with a small swing in between, instead of two icons
faded over each other. It sits on a disc of frosted artwork with a lit rim, and a
ring round it shows the track's progress, or runs while the track loads, which
replaces the flat spinner. The surface calms down under the icon gradually
instead of jumping flat, a double-tap no longer flickers the icon, and a mouse
resting on the playing drop shows the pause bars faintly.

Also:
- Low-density desktop screens render the drop at twice the resolution.
- Quality steps follow the display's own frame rate and step back up again.
- A lost WebGL context brings the drop back once it is restored.
- A paused drop keeps breathing at a low frame rate on desktop.
- Without Web Audio (iOS) jumps in the waveform count as kicks, so the drop lands on drops there too.
- Artists without an image get their own colour from their name, so their drop no longer sinks into the page.
- z toggles zen mode: just the drop, bigger, and the music.

web/index.html

@@ -190,6 +190,7 @@
         <dt>x</dt><dd>not for me</dd>
         <dt>s</dt><dd>share</dd>
         <dt>v</dt><dd>liquid / simple visuals</dd>
+        <dt>z</dt><dd>zen: just the drop</dd>
         <dt>m · + −</dt><dd>mute · volume</dd>
         <dt>?</dt><dd>this help</dd>
       </dl>