loop_type support

[AlexMitchellMus]

Support Loop_type opcode.

Allows sample looping to be:
• forward
• backward
• alternate

https://sfzformat.com/opcodes/loop_type.html

[kinwie]

This is supported in DropZone, Dimension and Rapture.
Sforzando parsed this silently and just set it to forward only.

[mathieugarcia]

Yup, I would love to see this one implemented.