The IJG README file has been renamed to README.ijg, in order to avoid
confusion (many people were assuming that that was our project's README
file and weren't reading README-turbo.txt) and to lay the groundwork for
markdown versions of the libjpeg-turbo README and build instructions.
Most of these involved left shifting a negative number, which is
technically undefined (although every modern compiler I'm aware of
will implement this by treating the signed integer as a 2's complement
unsigned integer-- the LEFT_SHIFT() macro just makes this behavior
explicit in order to shut up ubsan.) This also fixes a couple of
non-issues in the entropy codecs, whereby the sanitizer reported an
out-of-bounds index in the 4th argument of jpeg_make_d_derived_tbl().
In those cases, the index was actually out of bounds (caused by a
malformed JPEG image), but jpeg_make_d_derived_tbl() would have caught
the error and aborted prior to actually using the invalid address. Here
again, the fix was to make our intentions explicit so as to shut up
ubsan.
