GCC has some optimization for the inclusion guard, but they only work if
the check is the outermost one.
We're fairly inconsistent because of historical reasons, so we should
ensure that we follow the same pattern in every public header.
The plain C bytes array, while convenient from a C perspective, is not
well handled by language bindings: the length of the array is not
specified, and it's only just implied by the image data size, rowstride,
and pixel format.
GBytes is a read-only bytes buffer that has an implicit length; we can
use it as the storage medium so that language bindings can actually
function correctly.
