[1.14] Use the viewport-rel. cursor pos for CursorPosition (#13786)

In #13024, we removed `Terminal::GetCursorPosition` from TerminalCore.
This has been widely regarded as a good move.

Now, you might rightly be wondering: why didn't compilation immediately
fail? Well. It turns out that there were _two_ copies of
`GetCursorPosition`. One for `const Terminal`, and one for `Terminal`.
This is important.

`Terminal::GetCursorPosition()` returned the cursor position relative to
the viewport. `Terminal::GetCursorPosition() const`, however, returns
the cursor position in absolute.

We removed the non-`const` one. Fortunately, thanks to the lookup rules
for `const`-qualified members, this didn't matter. Code that called
`GetCursorPosition()` still called `GetCursorPosition()`, and everything
was fine.

Except that part about the relative coordinates. That was not fine.

The TSF control is the _only_ consumer of `ControlCore.CursorPosition`,
and that was the _only_ consumer of relative-`GetCursorPosition()`.

This commit restores equilibrium by introducing a new
`GetViewportRelativeCursorPosition()` member to `Terminal` and switching
over the only consumer of relative cursor position to use it.

Closes #13769.
Backport of #13785.

src/cascadia/TerminalControl/ControlCore.cpp

@@ -1419,7 +1419,7 @@ namespace winrt::Microsoft::Terminal::Control::implementation
         }
 
         auto lock = _terminal->LockForReading();
-        return til::point{ _terminal->GetCursorPosition() }.to_core_point();
+        return _terminal->GetViewportRelativeCursorPosition().to_core_point();
     }
 
     // This one's really pushing the boundary of what counts as "encapsulation".

src/cascadia/TerminalCore/Terminal.cpp

@@ -1462,3 +1462,12 @@ void Terminal::_updateUrlDetection()
         ClearPatternTree();
     }
 }
+
+// Method Description:
+// - Returns the position of the cursor relative to the active viewport
+til::point Terminal::GetViewportRelativeCursorPosition() const noexcept
+{
+    const til::point absoluteCursorPosition{ GetCursorPosition() };
+    const auto viewport{ _GetMutableViewport() };
+    return absoluteCursorPosition - til::point{ viewport.Origin() };
+}

src/cascadia/TerminalCore/Terminal.hpp

@@ -84,6 +84,8 @@ class Microsoft::Terminal::Core::Terminal final :
     bool IsXtermBracketedPasteModeEnabled() const;
     std::wstring_view GetWorkingDirectory();
 
+    til::point GetViewportRelativeCursorPosition() const noexcept;
+
     // Write comes from the PTY and goes to our parser to be stored in the output buffer
     void Write(std::wstring_view stringView);