#5412 modern-straight-flag and flat-flag should correspond to beams in thickness and gap

[Simon Albrecht]

Reported by edes:

For some reason, when using modern-straight-flag or flat-flag, flags are
much thicker than normal beams, and the distance is also much bigger.

Straight and flat flags were widely used by modern composers like Pierre
Boulez and Karlheinz Stockhausen, and in all cases flags were treated like
beams, with the same distance and the same width.

Please see the attached examples and images (I'm using 2.21.0 from git).

BTW, the first flag does not align with the end of the stem.

%%%% flat-flag.ly
\version "2.21.0"

\layout {
  \context {
    \Score
      \override Flag.stencil = #flat-flag
  }
}

\relative c' {
<<
  { 
    r8. d'16 d d d d  d r8 r32 d32 d[ d d d] d32 r16. }
  \\
  { r8. g,16 g g g g  g r8 r32 g32 g[ g g g] g32 r16. }
>>
}

%%%% modern-straight-flag.ly
\version "2.21.0"

\layout {
  \context {
    \Score
      \override Flag.stencil = #modern-straight-flag
  }
}

\relative c' {
<<
  { 
    r8. d'16 d d d d  d r8 r32 d32 d[ d d d] d32 r16. }
  \\
  { r8. g,16 g g g g  g r8 r32 g32 g[ g g g] g32 r16. }
>>
}

[Malte Meyn]

If I’m not mistaken the gap changes for more more than three beams to ensure that the beams cover the staff lines. This should be considered for flags too.