fluidsynth: update to 2.6.0

fluidsynth: update to 2.6.0

New features:
- automatic gain control / output limiter on the final rendering stage
- signed 32-bit and 24-bit linear PCM support; WASAPI driver now allows
  24-bit audio
- MIDI RPN 5 (Modulation Depth Range) support
- multiple reverb engines, addressing prior FDN reverb limitations
- GM2 bank select mode

Musically breaking changes:
- corrected a 17-year-old bug affecting sample fine-tuning in certain
  Soundfont instruments
- implemented MIDI RP-020 to fix MTS playback in MIDI files
- switched default reverb engine to Dattorro (plate-style)
- replaced 7th order sinc interpolation with a higher-order one due to
  quality concerns

Other: hardened Pipewire driver against xruns; exits with an error when
unable to load a command-line Soundfont; dropped the GLib and
libInstPatch dependencies.

Dropped +glib2 from DEPENDS to match: libfluidsynth.so.3.6.0 no longer
links libglib-2.0 (confirmed via readelf -d).

2.6.0 also drops the enable-libinstpatch CMake option entirely, so the
existing -Denable-libinstpatch=off pin became a silent no-op: DLS/GIG
loading is now controlled by the new enable-native-dls option instead,
which defaults to on. Left unpinned, this bump would have silently
re-enabled DLS/GIG support that this Makefile had deliberately turned
off. Replaced the dead pin with -Denable-native-dls=off to preserve
the existing behaviour explicitly.

Also pin the new enable-signalsmith option (limiter/AGC and the extra
reverb engines) to off: it defaults to on, but the feature it gates
depends on the signalsmith-audio-basics git submodule, which is not
present in the release tarball this package builds from - configure
silently compiles it out either way, so pinning it off just makes that
explicit and deterministic rather than accidental.

Signed-off-by: Daniel Golle <redacted>