FS#12189 - Simplify the codec API and don't always loop MODs.
This patch makes some changes to codec_api to help remove dependencies for my project.
The unused strcasestr callback is removed.
The global_settings pointer was being used by several codecs to check whether REPEAT_ONE mode is on; this has been replaced with a callback "should_loop()".
The MOD codec has been modified not to restart the file when it reaches the end. This does not affect MOD files that use "position jump" to loop part of the song. Ideally the MOD codec would only follow such loops if should_loop() returns 1, but detecting loops is nontrivial.
This task depends upon