Add support for Erase Color Mode (DECECM)

[j4james]

The Erase Color Mode determines what attributes are written to the
buffer when erasing content, or when new content is scrolled onto the
screen. When the mode is reset (which is the default), we erase with the
active colors, but with rendition attributes cleared. When the mode is
set, we erase with the default attributes, i.e. with neither color nor
rendition attributes applied.

This could be used to address the problem described in issue #10556.

Most of the affected operations are handled within the AdaptDispatch
class, so I've simply updated them all to use a new helper method which
returns the appropriate erase attributes for the active mode.

However, there were a couple of operations that are handled elsewhere,
and which now require the erase attributes to be passed to them as a
parameter.

• The TextBuffer::IncrementCircularBuffer method, which is used to
  recycle the topmost row when scrolling past the bottom of the buffer.

• The TextBuffer::SetCurrentLineRendition method, which has to clear
  the second half of the line when switching to a double width rendition.

• The ITerminalApi::UseAlternateScreenBuffer method, which has to
  clear the screen when switching to the alternate buffer.

Then there is also a Clear Buffer action in Windows Terminal, which is
ultimately handled by the SCREEN_INFORMATION::ClearBuffer method in
ConHost. That class has no access to the erase color mode, so has no way
of knowing which attributes to use. So I've now rewritten it to use the
AdaptDispatch::EraseInDisplay method to handle the erasing.

Validation Steps Performed

I wrote a little test script that exercises the operations affected by
DECECM, which @al20878 kindly tested for us on a real DEC VT525, and
provided screenshots of the output. I've manually confirmed that our
implementation exactly matches those results.

I've also added a unit test that runs through the same set of operations
and verified that each of them is using the appropriate attributes when
DECECM is enabled and enabled.

Closes #14983

[j4james]

Some screenshots for comparison with the VT525 output in #14983 (comment).

[carlos-zamora]

Seriously reviewed the whole thing and not a single comment haha. Thanks for doing this!

[lhecker]

BTW I was reading what EraseInDisplay does and it'd be nice if we could replace the _FillRect calls that operate on entire rows with ones that call ROW::Reset() instead. With the upcoming PR #15501 this would be much much faster. With #15524 it'd be additionally curious if we could instead just decommit the entire cleared memory area, for instance when we encounter a sequence to clear the scrollback. (Just thinking out loud.)

[j4james]

BTW I was reading what EraseInDisplay does and it'd be nice if we could replace the _FillRect calls that operate on entire rows with ones that call ROW::Reset() instead.

Yeah, that sounds like a good idea. _EraseScrollback in particular should be a big win. But note that it doesn't clear the full height of the buffer - you still need to retain one viewport worth of content - so we couldn't just nuke the entire thing.

[DHowett]

I've kicked the test job in CI :)