Bugfix DECRQM (Dec Request Mode) response

[jquast]

Interactive terminal sequence response for DEC Private Mode Query is invalid -- it may respond with a different mode and status than the one requested.

This causes programs to fail or mismatch contour's terminal capabilities. In my API I was expecting the requested mode value in the response and, not matching it for any modes under number 27, wrongly reported that Contour does not support any DEC Private Modes at all.

This bug is present in all versions of Contour supporting DEC Private Mode, since 256e313 and refactored in faa66e7, the use of:

   static_cast<DECMode>(mode)

Is invalid, because the DECMode enum has "holes". It is not contiguous.

A new fromDECModeNum() function is provided, refactored form of isValidDecMode() -- returning an optional DecMode, existing function isValidDECMode() is refactored to check whether the optional value returned by 'fromDECModeNum' tests true for has_value().

Given the provided test file test/display-modes.sh, we can make inquiry of DEC Private modes and see that DEC modes 1 through 27 are returned with non-matching response modes mismatched by enum item order and status:

$ ./test/display-modes.sh
Query mode 1: response=$'\E[?2;2$y' after 13ms
Query mode 2: response=$'\E[?3;2$y' after 11ms
Query mode 3: response=$'\E[?4;2$y' after 10ms
Query mode 4: response=$'\E[?5;2$y' after 11ms
Query mode 5: response=$'\E[?9;2$y' after 11ms
Query mode 6: response=$'\E[?1000;2$y' after 12ms
Query mode 7: response=$'\E[?1001;2$y' after 13ms
Query mode 8: response=$'\E[?1002;2$y' after 13ms
Query mode 9: response=$'\E[?1003;2$y' after 12ms
Query mode 10: response=$'\E[?1048;2$y' after 14ms

After this bugfix, this is corrected:

$ ./test/display-modes.sh
Query mode 1: response=$'\E[?1;2$y' after 11ms
Query mode 2: response=$'\E[?2;2$y' after 11ms
Query mode 3: response=$'\E[?3;2$y' after 8ms
Query mode 4: response=$'\E[?4;2$y' after 10ms
Query mode 5: response=$'\E[?5;2$y' after 8ms
Query mode 6: response=$'\E[?6;2$y' after 11ms
Query mode 7: response=$'\E[?7;1$y' after 11ms
Query mode 8: response=$'\E[?8;0$y' after 10ms
Query mode 9: response=$'\E[?9;2$y' after 10ms
Query mode 10: response=$'\E[?10;2$y' after 12ms

Further, looking at other uses of isValidDECMode(), some were unnecessary -- that they have the DECMode type bound to the variable is sufficient enough to describe that mode as valid, the check is superfluous and removed, eg:

  isValidDECMode(static_cast<unsigned int>(mode)));

the call to toDECModeNum is also superfluous and removed.

[christianparpart]

Many thanks for the fix. Is it okay to apply the minors, for you?

[christianparpart]

Thank you very much for your contribution, @jquast 😄