Added vowel transitions for lyrics

input/regression/vowel-transition-minimum-length.ly

+\version "2.21.0"
+
+\header {
+
+  texidoc = "For vowel transitions, @code{minimum-length} refers to
+the drawn length of the arrow.  The protrusion of the syllables and
+padding is in effect added to @code{minimum-length} for spacing.  This
+default behaviour can be changed by overriding @code{springs-and-rods},
+which may cause the transition arrow not to be drawn if there is
+insufficient space (rather than adding the space necessary to draw it
+at @code{minimum-length}).  @code{minimum-length-after-break} controls
+the minimum length of the segment following a system break."
+
+}
+
+\layout {
+  ragged-right = ##t
+}
+
+\score {
+  {
+    \relative {
+      c'2( ^\markup {
+        \left-column { \tiny { Padding increases spacing } } } c) |
+      c( c) |
+      c( ^\markup {
+        \left-column { \tiny { Padding shortens arrow } } } c) |
+      c( c) |
+      c4 ^\markup {
+        \left-column { \tiny { "Not enough space"
+                               "to draw arrow at"
+                               "minimum-length." } } } c r2 | \break
+      c2 ^\markup {
+        \left-column { \tiny { "Spacing increased"
+                               "by extent of"
+                               "bounds protrusion" } } } c |
+      c ^\markup {
+        \left-column { \tiny { "No space added"
+                               "to compensate for"
+                               "bounds protrusion."
+                               "Not enough space"
+                               "to draw arrow at"
+                               "minimum-length." } } } c \bar "||" \break
+      c1 ^\markup { \tiny { "minimum-length-after-break only applies..." } }
+      c1 \break
+      c1 ^\markup { \tiny { "...after a system break." } }
+    }
+
+    \addlyrics {
+      \override VowelTransition.minimum-length = #7
+      \override VowelTransition.bound-details.left.padding = #4
+      \override VowelTransition.bound-details.right.padding = #4
+      a \vowelTransition b
+      \temporary \override VowelTransition.springs-and-rods = #ly:spanner::set-spacing-rods
+      c \vowelTransition d
+      e \vowelTransition f
+      \revert VowelTransition.springs-and-rods
+      eeeee \vowelTransition fffff
+      \once \override VowelTransition.springs-and-rods = ##f
+      ggggg \vowelTransition hhhhh
+      i \vowelTransition
+      \override VowelTransition.after-line-breaking = ##t
+      \override VowelTransition.minimum-length-after-break = #20
+      jjjjjjjjjjjjjjjjjjjj \vowelTransition
+      kkkkkkkkkkkkkkkkkkkk
+    }
+  }
+}