This avoids the 404 errors that would previously result.

A compact encoding is used, with `!` plus the face letter used to specify a
new face, `>` plus a base83-encoded number used to specify a new level, and a
list of base83-encoded numbers used to specify x and y tile coordinates, where
the base83 encoding uses the minimum number of characters to encode the
maximum tile number for the level in question. The base83 encoding is the same
as the one used for SHT hashes.

Latest version of pyshtools uses Meson for building and does not build
in 20.04.

Previously, tiles with fewer than four children were always drawn, even if
all their children were also drawn.

The optimizations introduced in 05dae19b28
contained a bug, which caused a regression.

It now no longer works with Python <3.2.

While the last commit prevented a race condition, it also broke the
`updateOnce` function.

Thanks to @strarsis.

This is a result of browsers' stricter protections around accessing device
sensors.

This was an issue in Chrome >= 70.

Thanks to @KUJensen for suggesting the subsequent-sibling combinator as a fix.

As iPads use the same user agent as Mac, we need to check if touch input is
supported instead.

This allows interoperability with other WebGL code.

If the viewer is destroyed, we now try to abort loading equirectangular and
cube map panoramas. This appears to work in Chrome but not in Firefox.

No attempt is made to stop loading multires images, since that process is more
complicated.

Previously, a single multires tile level was used for the entire viewport,
which resulted in too much resolution at the center and too little at the
edges. This was particularly noticable when zoomed out all the way.

The new approach selectively loads tiles based on their individual projected
resolution, which is more efficient and should resolve long-standing
edge-softness issues.

It was added in 950942830fb531bf8faee19f85dea5cc94a23de1, but needs more work.