[Leonard Hecker (lhecker)]

This commit fixes 3 bugs:

• COOKED_READ_DATA failed to initialize its _distanceCursor and
  _distanceEnd members. I took this as an opportunity to make them
  ptrdiff_t, to reduce the likelihood of overflows in the future.
• COOKED_READ_DATA::_writeChars added scrollY to the written
  distance, even though WriteCharsLegacy writes a negative value into
  that out parameter. This was fixed by changing WriteCharsLegacy to
  write positive values and by adding a debug assertion.
• StreamScrollRegion calls IncrementCircularBuffer which causes a
  synchronous (!) ConPTY flush to the output pipe (side note: this is
  the primary reason why newlines are so slow in ConPTY).
  Since cooked reads are supposed to behave like a pager and not write
  into the scrollback, we temporarily mark the buffer as inactive
  which prevents TextBuffer from snitching about it to VtEngine.

Even after this change, there's still some weird behavior left:

• You cannot move your cursor back beyond (0,0), because this isn't a
  real pager-like implementation. That might be a neat future extension.
• Writing a lot of text and pressing Ctrl+C doesn't properly place the
  cursor and scroll the buffer, unless the cursor is at the end.
  That might also be worth investigating in the future (minor issue).
• When the viewport is full, backspacing more than 1 line of text
  (using Ctrl+Backspace) doesn't erase all of the affected lines,
  because COOKED_READ_DATA::_erase uses the same WriteCharsLegacy
  function to write whitespace to erase that text. It's only gone
  after typing one more character.
  I've written the code to mostly fix this, but decided against it
  as I considered the problem to be too niche to warrant extra code.

Closes #15899

Validation Steps Performed

• Generate some text to paste in PowerShell:

  "" + (0..512 | % { "word" + $_.ToString().PadLeft(4, "0") })

• Launch cmd.exe and paste that text
• No flickering ✅
• No writing into the scrollback ✅
• No weird behavior when backspacing ✅

[Leonard Hecker (lhecker)]

This makes the tests pass. I don't think they ever relied on the interactive behavior of WriteCharsLegacy.

[Leonard Hecker (lhecker)]

After a lot of back and forth, I went with ptrdiff_t after all. That way we can have bigger buffers in the future and make full use of our int32_t coordinates.

[Leonard Hecker (lhecker)]

This helped me find the 2nd bug.

[Leonard Hecker (lhecker)]

Whoops. Forgot to 0 initialize.