The optimization silences the warning about type limits on platforms with `char` type as `unsigned char`. The original condition is semantically identical to the optimized one because the signed-to-unsigned cast is well-defined in C standard.
(This commit also modifies deps/libcaption) Closes obsproject/obs-studio#1354
Manually merging/squashing from: https://github.com/szatmary/libcaption
