Cache VT buffer line string to avoid (de)alloc on every paint (#6840)

A lot of time was spent between each individual line in the VT paint
engine in allocating some scratch space to assemble the clusters then
deallocating it only to have the next line do that again. Now we just
hold onto that memory space since it should be approximately the size of
a single line wide and will be used over and over and over as painting
continues.

## Validation Steps Performed
- Run `time cat big.txt` under WPR. Checked before and after perf
  metrics.

src/renderer/vt/paint.cpp

@@ -334,17 +334,17 @@ using namespace Microsoft::Console::Types;
     {
         RETURN_IF_FAILED(_MoveCursor(coord));
 
-        std::wstring wstr;
-        wstr.reserve(clusters.size());
+        _bufferLine.clear();
+        _bufferLine.reserve(clusters.size());
 
         short totalWidth = 0;
         for (const auto& cluster : clusters)
         {
-            wstr.append(cluster.GetText());
+            _bufferLine.append(cluster.GetText());
             RETURN_IF_FAILED(ShortAdd(totalWidth, gsl::narrow<short>(cluster.GetColumns()), &totalWidth));
         }
 
-        RETURN_IF_FAILED(VtEngine::_WriteTerminalAscii(wstr));
+        RETURN_IF_FAILED(VtEngine::_WriteTerminalAscii(_bufferLine));
 
         // Update our internal tracker of the cursor's position
         _lastText.X += totalWidth;
@@ -371,21 +371,21 @@ using namespace Microsoft::Console::Types;
         return S_OK;
     }
 
-    std::wstring unclusteredString;
-    unclusteredString.reserve(clusters.size());
+    _bufferLine.clear();
+    _bufferLine.reserve(clusters.size());
     short totalWidth = 0;
     for (const auto& cluster : clusters)
     {
-        unclusteredString.append(cluster.GetText());
+        _bufferLine.append(cluster.GetText());
         RETURN_IF_FAILED(ShortAdd(totalWidth, static_cast<short>(cluster.GetColumns()), &totalWidth));
     }
-    const size_t cchLine = unclusteredString.size();
+    const size_t cchLine = _bufferLine.size();
 
     bool foundNonspace = false;
     size_t lastNonSpace = 0;
     for (size_t i = 0; i < cchLine; i++)
     {
-        if (unclusteredString.at(i) != L'\x20')
+        if (_bufferLine.at(i) != L'\x20')
         {
             lastNonSpace = i;
             foundNonspace = true;
@@ -479,8 +479,7 @@ using namespace Microsoft::Console::Types;
     RETURN_IF_FAILED(_MoveCursor(coord));
 
     // Write the actual text string
-    std::wstring wstr = std::wstring(unclusteredString.data(), cchActual);
-    RETURN_IF_FAILED(VtEngine::_WriteTerminalUtf8(wstr));
+    RETURN_IF_FAILED(VtEngine::_WriteTerminalUtf8({ _bufferLine.data(), cchActual }));
 
     // GH#4415, GH#5181
     // If the renderer told us that this was a wrapped line, then mark

src/renderer/vt/state.cpp

@@ -49,7 +49,8 @@ VtEngine::VtEngine(_In_ wil::unique_hfile pipe,
     _newBottomLine{ false },
     _deferredCursorPos{ INVALID_COORDS },
     _inResizeRequest{ false },
-    _trace{}
+    _trace{},
+    _bufferLine{}
 {
 #ifndef UNIT_TESTING
     // When unit testing, we can instantiate a VtEngine without a pipe.

src/renderer/vt/vtrenderer.hpp

@@ -204,6 +204,9 @@ namespace Microsoft::Console::Render
 
         bool _WillWriteSingleChar() const;
 
+        // buffer space for these two functions to build their lines
+        // so they don't have to alloc/free in a tight loop
+        std::wstring _bufferLine;
         [[nodiscard]] HRESULT _PaintUtf8BufferLine(std::basic_string_view<Cluster> const clusters,
                                                    const COORD coord,
                                                    const bool lineWrapped) noexcept;