Enable using the alt buffer in the Terminal (#12561)

This PR allows the Terminal to actually use the alt buffer
appropriately. Currently, we just render the alt buffer state into the
main buffer and that is wild. It means things like `vim` will let the
user scroll up to see the previous history (which it shouldn't).

Very first thing this PR does: updates the
`{Trigger|Invalidate}Circling` methods to instead be
`{Trigger|Invalidate}Flush(bool circling)`. We need this so that when an
app requests the alt buffer in conpty, we can immediately flush the
frame before asking the Terminal side to switch to the other buffer. The
`Circling` methods was a great place to do this, but we don't actually
want to set the circled flag in VtRenderer when that happens just for a
flush. 

The Terminal's implementation is a little different than conhost's.
Conhost's implementation grew organically, so I had it straight up
create an entire new screen buffer for the alt buffer. The Terminal
doesn't need all that! All we need to do is have a separate `TextBuffer`
for the alt buffer contents. This makes other parts easier as well - we
don't really need to do anything with the `_mutableViewport` in the alt
buffer, because it's always in the same place. So, we can just leave it
alone and when we come back to the main buffer, there it is. Helper
methods have been updated to account for this. 

* [x] Closes #381
* [x] Closes #3492
* #3686, #3082, #3321, #3493 are all good follow-ups here.

src/buffer/out/textBuffer.cpp

@@ -562,7 +562,7 @@ bool TextBuffer::IncrementCircularBuffer(const bool inVtMode)
     // to the logical position 0 in the window (cursor coordinates and all other coordinates).
     if (_isActiveBuffer)
     {
-        _renderer.TriggerCircling();
+        _renderer.TriggerFlush(true);
     }
 
     // Prune hyperlinks to delete obsolete references

src/cascadia/TerminalCore/ITerminalApi.hpp

@@ -69,6 +69,9 @@ namespace Microsoft::Terminal::Core
         virtual void PushGraphicsRendition(const ::Microsoft::Console::VirtualTerminal::VTParameters options) = 0;
         virtual void PopGraphicsRendition() = 0;
 
+        virtual void UseAlternateScreenBuffer() = 0;
+        virtual void UseMainScreenBuffer() = 0;
+
     protected:
         ITerminalApi() = default;
     };

src/cascadia/TerminalCore/Terminal.hpp

@@ -138,6 +138,8 @@ class Microsoft::Terminal::Core::Terminal final :
     void PushGraphicsRendition(const ::Microsoft::Console::VirtualTerminal::VTParameters options) override;
     void PopGraphicsRendition() override;
 
+    void UseAlternateScreenBuffer() override;
+    void UseMainScreenBuffer() override;
 #pragma endregion
 
 #pragma region ITerminalInput
@@ -319,11 +321,11 @@ class Microsoft::Terminal::Core::Terminal final :
     SelectionExpansion _multiClickSelectionMode;
 #pragma endregion
 
-    // TODO: These members are not shared by an alt-buffer. They should be
-    //      encapsulated, such that a Terminal can have both a main and alt buffer.
-    std::unique_ptr<TextBuffer> _buffer;
+    std::unique_ptr<TextBuffer> _mainBuffer;
+    std::unique_ptr<TextBuffer> _altBuffer;
     Microsoft::Console::Types::Viewport _mutableViewport;
     SHORT _scrollbackLines;
+    bool _detectURLs{ false };
 
     // _scrollOffset is the number of lines above the viewport that are currently visible
     // If _scrollOffset is 0, then the visible region of the buffer is the viewport.
@@ -375,6 +377,10 @@ class Microsoft::Terminal::Core::Terminal final :
 
     void _NotifyTerminalCursorPositionChanged() noexcept;
 
+    bool _inAltBuffer() const noexcept;
+    TextBuffer& _activeBuffer() const noexcept;
+    void _updateUrlDetection();
+
 #pragma region TextSelection
     // These methods are defined in TerminalSelection.cpp
     std::vector<SMALL_RECT> _GetSelectionRects() const noexcept;