Cargo 1.56 is needed for cargo-c but 1.55 is in U18 runner ver
20211017.0 thus update as a workaround.

Failed run:
https://github.com/MartinPulec/UltraGrid/actions/runs/1412342915

As there will be releases 1.7.1, 1.7.2 etc., storing zsync in release
asset doesn't make sense (further version won't update previous update
information). Thus store this in continuous release assets - update
channel "release" was introduced aside to existing "continuous".

The release channel is not build from release/ branch, instead a bugfix
version numbers are required and the build is from appropriate tag, eg.
v1.7 (release, should be in master) and then v1.7.1, v1.7.2. The bugfix
releases should have its own entry in NEWS file (create-release.sh makes
GitHub release description from this).

May be used for quick check if not running an obsolete UG during
runtime.

(related to previous commmit)

Originally mute for both sender and receiver were dispatched by the
sender, which doesn't need to be present, however.

+ added SPOUT to Syphon for hide_window use case

Adding of displayed and corrupted frames was accidentaly removed by a779d96c.

This function may not be used at all but just to make sure.

- chech return value of getnatpmprequesttimeout()
- break the loop after 5 seconds (to avoid infinite loop)

It isn't double-released, anyway, but just for sure.

Smpte bars doesn't have the eight colum as EBU has.

Moreover the pattern is not complete - the bottom third contains
different data - mention this in help + todo.

GCC 11 gives incorrect warning for unintialized strerror_buf on
strerror_buf.size(), which is, however, compile-time constant. As a
workaround a separate size constant is used.

When uncompressed it requires less bandwidth and it is more suitable for
compression and/or further processing.

+ fail if audio BPS or sample rate is invalid rather than displaying
  only a warning
+ removed unused FRAME_TIMEOUT

Incorrect BPP (3) was used for RGBA

- use $(CXX) instead of hard-codec g++ in Makefile.convert
- conversion -> list-conversions + brief usage