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.

This lets us use the code for non-float variable types. I also got
rid of the mat4-to-quat code so that users are forced to use the
mat3-to-quat code and possibly discover places where their code is
suboptimal.