nullsound: reworked slide FX semantics

Refactored the three slide FX (note slide, pitch slide, portamento)
and the transition between slides and notes, to better match the
semantics from Furnace.

Captured various examples in a doc to detail the expected behaviour
of each slide FX.

Author: dciabrin

nullsound/doc/slide.md

@@ -0,0 +1,69 @@
+# Slide FX semantics
+
+## pitch slide (01xx / 02xx)
+
+Move current note up or down
+
+  - FX specifies a direction (up / down), and a decimal increment
+  - slide stops when explicitely disabled (01.. / 02..) or when another slide FX is configured
+  - slide is relative to the note currently in use
+  - As soon as the FX is applied, it adds to the current note's decimal position
+  - When changing current note when slide is in progress, the FX continues from this new current note's decimal position
+
+```
+E-40A..0101
+...........
+...........
+...........
+...........
+D-40A......
+```
+
+## portamento (03xx)
+
+Move current note up or down towards a target note
+
+  - FX specifies a speed only, and assumes the target note is the note specified in the score's current line
+  - portamento continues until the target note is reached, or when another slide FX is configured
+  - portamento direction (up/down) is determined based on the current note
+  - When changing current note while portamento is in progress, this only changes the portamento target (and thus the portamento direction)
+
+ex: portamento from C5 to G-5
+
+```
+C-50A......
+G-50A..0301
+...........
+...........
+...........
+...........
+```
+
+ex: portamento from C5 to G5, increase speed to target after started
+
+```
+C-50A......
+G-50A..0301
+...........
+...........
+G-50A..0308
+...........
+```
+
+## note slide (E1xx / E2xx)
+
+Move current note up or down by a number of semitones
+
+  - FX is effectively a portamento FX whose target is relative to the note currently in use
+  - The remaining semantics is the same as a portamento effect
+
+ex: slide from A-5 to B-5, and set next note to B-5 (end will be B-5 after portamento finishes)
+
+```
+A-50A......
+...........
+...........
+.......E122
+B-50A......
+...........
+```