By writing the fxc and dxc output to a temporary file and then
converting that temporary file to the desired filename, we avoid
the incompatible semantics of sed -i on GNU systems (sed -i does not
create a backup filename, and does not take an argument unless it is
"bundled") and macOS (sed -i requires an argument, possibly empty).
Resolves: https://github.com/libsdl-org/SDL/issues/10878
Signed-off-by: Simon McVittie <smcv@debian.org>
