A_StartSound's startTime - Additional flags

[Major Cooke]

It may be worth having a special value (-1?) that starts the sound at the sound's LOOP_START marker (https://zdoom.org/wiki/Audio_loop)

From this post, I agree with it but I'd probably have flags instead. These would manipulate startTime to scale based on which is used.

• SSF_LOOP_START - Starts where the LOOP_START meta is defined. Adapts the startTime so 0 is where LOOP_START begins.
• SSF_CLIP_TO_LOOP_END - If LOOP_END is included on the sound, then scales startTime's end (1.0) to LOOP_END.
• SSF_LOOP_END - Starts where the LOOP_END meta is defined, so startTime at 0 would start there and 1.0 would be at the end of the sound file.