Re-enable export file tagging. No idea why this code has dropped out at some stage...
Fix export format dialog tagging state inconsistency

The state of a checkbox was used before the UI was properly initialized.
This always unset the tagging option in the format object.
initialize some uninitialized variables
add support for old compilers w/o implicit stdbool
centralize buffer silent-flag

fixes possible x-talk 1 in, >= 2 out tracks:

Previously, only the first route-buffer of the input buffers
were marked as non-silent in Route::process_output_buffers().
Other buffers in the set (e.g. post-panner) would
contain audio but not marked as non-silent.
remove unused API to explicitly set/unset buffer silent flag
prevent 0-pointer dereference in editor_routes.cc - fixes #5794
Merge branch 'master' into cairocanvas