* Expose API via Public Editor
* Skip hidden tracks in calculation
* Use a single call to query playlist extents.
This way the playlist read-lock is needed at most once per track
which speeds up calculation.
Various playlist operations can change region-properties
which results in Region::send_change being emitted while
the Playlist::RegionWriteLock is held.
This can result in recursive lock and/or deadlocks or crashes. e.g.
Insert time -> Playlist::shift -> Region::RegionPropertyChanged
-> EditorSummary::set_background_dirty -> Editor::session_gui_extents
-> Playlist::get_extent -> read-lock is taken after write-lock.
This is in preparation to subscribe to playhead cursor position
changes in the recorder-UI.
This change also clean up the API, replacing a public variable
with a const access method and follows #12 of
https://ardour.org/styleguide.html
FoldbackStrip::update_send_box() GUI relies on ->fed_by() to
determine if a FB bus has a send from a route.
Under the hood this uses direct_feeds_according_to_reality()
and InternalSend::feeds(). When `allow_feedback` is set a send is
assumed to not directly feed a target (to allow loopbacks).
This mode must not be used for foldback sends.
This fixes i386 optimized builds:
```
dyld: Symbol not found: _PL_CompareValues
Referenced from: /Volumes/Ardour-6.5.86/Ardour6.app/Contents/MacOS/../lib/libnssutil3.dylib
Expected in: /Volumes/Ardour-6.5.86/Ardour6.app/Contents/MacOS/../lib/libplds4.dylib
in /Volumes/Ardour-6.5.86/Ardour6.app/Contents/MacOS/../lib/libnssutil3.dylib
```
Older compilers do not yet have fmaintrin.h. This fixes compile errors:
```
x86_functions_fma.cc:51:39: error: '_mm_fmadd_ss' was not declared in this scope
x86_functions_fma.cc:85:35: error: '_mm256_fmadd_ps' was not declared in this scope
x86_functions_fma.cc:106:35: error: '_mm256_fmadd_ps' was not declared in this scope
x86_functions_fma.cc:129:39: error: '_mm_fmadd_ss' was not declared in this scope
```
By supporting FMA extension, the number of instruction needed
for multiply accumulate to mix channels are reduced. Since,
this extension has been around since middle of 2012, more
computers have this instruction set available.
