Use the viewport-relative cursor position for CCore.CursorPosition

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.

src/cascadia/TerminalControl/ControlCore.cpp

@@ -1602,7 +1602,7 @@ namespace winrt::Microsoft::Terminal::Control::implementation
         }
 
         auto lock = _terminal->LockForReading();
-        return _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

@@ -1638,3 +1638,12 @@ til::color Terminal::GetColorForMark(const Microsoft::Console::VirtualTerminal::
     }
     }
 }
+
+// Method Description:
+// - Returns the position of the cursor relative to the active viewport
+til::point Terminal::GetViewportRelativeCursorPosition() const noexcept
+{
+    const auto absoluteCursorPosition{ GetCursorPosition() };
+    const auto viewport{ _GetMutableViewport() };
+    return absoluteCursorPosition - 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);