Commit 738ac0dc01 increased the size of the ImageInner::StaticTextures variant,
which breaks the size assertions in the interpreter ffi build.
This changes fixes that by reducing the size of the ImageInner variant again by
moving all the fields into a separate struct.
Since the cat picture is now in jpeg, we need to be able to decaode it
from the tests.
Luckily on the CI this works fine beacause we do a full cargo test that
build all the crates including the gl crate that has the jpeg format enaabled,
but if we try to compile the test with limited feature set, it wouldn't be available.
The README.md contains the warning that used to be in lib.rs.
Add README.md files to all internal crates
... pointing to the official public crate to use instead.
Rename internal crates
fixup: README files
fixup rename
