Add String.split(char) to split into an array, and add a simple
char const * + lol::String free operator.

There might be issues with lolremez but at first sight it seems to work.

This doesn’t work properly yet because PICO-8 uses a few language
extensions; we will have to decide whether to modify the Lua
interpreter, or change the code on the fly, or something entirely
different.

For now, all it can do is read and decompress .p8.png files.

Various changes in GCC, clang, and libflex had triggered build failures.

If SDL_CFLAGS is defined to a non-empty value, even spaces, a later
test will wrongly succeed and think SDL_CFLAGS is a user override.

This avoids silly mistakes when calling radians(180) instead of
radians(180.0). The first one used to return 3 instead of 3.14159…

We already have the convenient degrees() and radians() functions to convert
between angle formats. This commit involves a lot of refactoring here and
there and I may have missed some places where conversions were needed. But
hopefully there aren’t may such places.

The name “re” came from “reciprocal” but since we have “inverse” for
matrices, I thought it would be nice to make everything consistent.

This is to avoid confusion with Scene::AddTile().

The old public API is maintained for now but should be considered
deprecated.