It is only read-from, so make it const. In order to be able to do this,
constify all places where mxc_jpeg_fmt is used, in function arguments,
return values and pointers. On top of that, make the name a pointer to
const char.
On aarch64, this shrinks object code size with 550 bytes with gcc 11.1.0,
and almost 2kB with clang 12.0.0.