#1161)

* softcut: buffer copy with reverse

* add crossfades and stereo / mono copy methods

* rendering buffer content working, with some bugs

* fix some buffer rendering cases / function signatures

* clean up softcut.buffer_render luadoc

* raised cosine fade at both ends of copied material

* add preserve option for mixing existing content in region

* fix mixing calculation error in reverse case

* fix several embarrassing bounds checking mistakes

* handle overlapping regions for non-reversed copy

Co-authored-by: Sam Boling <csboling@noreply.github.com>