ConPTY: Avoid WINDOW_BUFFER_SIZE_EVENT when the viewport moves (#15935)

`SetConsoleWindowInfoImpl` calls `PostUpdateWindowSize`, which emits a
`CM_SET_WINDOW_SIZE` event, which causes `_InternalSetWindowSize` to be
called, which calls `ScreenBufferSizeChange` which then finally emits a
`WINDOW_BUFFER_SIZE_EVENT` event into the client input buffer.

This messes up applications like which make use of
`WINDOW_BUFFER_SIZE_EVENT` to perform potentially lossy operations.
In case of SSH this results in a resize (SIGWINCH) of the server-side
screen which similarly may result in a response by the shell, etc.
Since that happens over networks and is async, and because our conhost
VT viewport implementation appears to have a number of subtle bugs,
this results in duplicate output lines (sometimes hundreds).

Under Windows Terminal this issue is not as apparent, since ConPTY has
no viewport that can be moved and no scrollback. It only appears as an
issue if a terminal application reacts poorly to the SIGWINCH event.

Closes #15769

## Validation Steps Performed
* Set a breakpoint in `SynthesizeWindowBufferSizeEvent`
* Launch WSL and cause the viewport to move down
  No calls to `SynthesizeWindowBufferSizeEvent` ✅
* Execute `tput reset`
  Input line moves to row 0 ✅

.github/actions/spelling/expect/expect.txt

@@ -1908,6 +1908,7 @@ touchpad
 Tpp
 Tpqrst
 tprivapi
+tput
 tracelog
 tracelogging
 traceloggingprovider

src/host/outputStream.cpp

@@ -95,9 +95,11 @@ til::rect ConhostInternalGetSet::GetViewport() const
 void ConhostInternalGetSet::SetViewportPosition(const til::point position)
 {
     auto& info = _io.GetActiveOutputBuffer();
-    const auto dimensions = info.GetViewport().Dimensions();
-    const auto windowRect = til::rect{ position, dimensions }.to_inclusive_rect();
-    THROW_IF_FAILED(ServiceLocator::LocateGlobals().api->SetConsoleWindowInfoImpl(info, true, windowRect));
+    THROW_IF_FAILED(info.SetViewportOrigin(true, position, false));
+    // SetViewportOrigin() only updates the virtual bottom (the bottom coordinate of the area
+    // in the text buffer a VT client writes its output into) when it's moving downwards.
+    // But this function is meant to truly move the viewport no matter what. Otherwise `tput reset` breaks.
+    info.UpdateBottom();
 }
 
 // Method Description: