(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

fix type o found to find

+ small improvements (use of MAX_PATH_SIZE)

SDL, GL and NDI required mainloop only on macOS.

The export structure was not zero-initialized as expected when checked
for NULL.

Use an user-friendlier error message than assert.