This partially reverts 2a511d58. The value 4 should be default now (best
quality), BPS==2 should be perhaps set later by a preset (after/if
implemented).

Added stub allowing dummy decode (without error correction) for the
receiver basic run.

+ do not abort on unknown PT in audio

Clarify that PT_JPEG isn't our JPEG but RFC-compliant.

Support FPS values that are not understood by UG (FPSd == 0) and
replace it by 30.

This is a forward compatibility feature when there may be special meaning FPS
values (unlimited etc.) but it can still be displayed, especially by displays
that don't honor FPS value like OpenGL.

- fixed [,] special meaning in M4
- qmake version >6 -> >=6
- typo: QMAKE_VER instead of QMAKE_VERSION

For Linux AppImages in release channel, zsync files are stored as an
artefact in continuous release assets so ensure that it is present.

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.