#10477: Handle things above U+FFFF in GDI renderer (#10580)

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## Implementation of #10477 - handle surrogate pairs in GDI renderer.

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## PR Checklist
* [ ] Closes #10477
* [x] CLA signed.
* [ ] Tests added/passed
* [ ] Documentation updated.
* [ ] Schema updated.
* [x] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #10477

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## Detailed Description of the Pull Request / Additional comments
### Why not let Windows draw surrogate pairs? It can do that.

Basically, the comment says everything:
https://github.com/microsoft/terminal/blob/c90de692509b074bfde191910d67154cfe389911/src/renderer/gdi/paint.cpp#L346-L347

However, handling things above U+FFFF doesn't really require extra effort. It's enough to:
- Put *all* characters to the output buffer
- Set the first width to cluster width and the rest to 0
- Sit back and relax while Windows does the rest

<!-- Describe how you validated the behavior. Add automated tests wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed
```CMD
@echo off
chcp 65001

echo 𠜎𠝹𠱓𠱸𠲖𠳏𠳕𠴕𠵼𠵿𠸎
echo 👨👩👧👦
```
Save this as a UTF-8 cmd file and run.

### Before the change
![image](https://user-images.githubusercontent.com/11453922/122832196-ed438880-d2e2-11eb-93dd-931954efedbf.png)

### After the change
![image](https://user-images.githubusercontent.com/11453922/122832217-f2a0d300-d2e2-11eb-99f0-e129e5544667.png)

An example of a third party app working with surrogate pairs in a patched OpenConsole:
![image](https://user-images.githubusercontent.com/11453922/122838225-837cac00-d2ed-11eb-8faf-dbeb52f77916.png)

As discussed, this change doesn't claim to be the full support for surrogate pairs (there are still corner cases possible), but brings it on par with Terminal with minimal effort.

src/renderer/gdi/paint.cpp

@@ -328,11 +328,13 @@ using namespace Microsoft::Console::Render;
 
         const auto pPolyTextLine = &_pPolyText[_cPolyText];
 
-        auto& polyString = _polyStrings.emplace_back(cchLine, UNICODE_NULL);
+        auto& polyString = _polyStrings.emplace_back();
+        polyString.reserve(cchLine);
 
         COORD const coordFontSize = _GetFontSize();
 
-        auto& polyWidth = _polyWidths.emplace_back(cchLine, 0);
+        auto& polyWidth = _polyWidths.emplace_back();
+        polyWidth.reserve(cchLine);
 
         // Sum up the total widths the entire line/run is expected to take while
         // copying the pixel widths into a structure to direct GDI how many pixels to use per character.
@@ -343,11 +345,11 @@ using namespace Microsoft::Console::Render;
         {
             const auto& cluster = til::at(clusters, i);
 
-            // Our GDI renderer hasn't and isn't going to handle things above U+FFFF or sequences.
-            // So replace anything complicated with a replacement character for drawing purposes.
-            polyString[i] = cluster.GetTextAsSingle();
-            polyWidth[i] = gsl::narrow<int>(cluster.GetColumns()) * coordFontSize.X;
-            cchCharWidths += polyWidth[i];
+            const auto text = cluster.GetText();
+            polyString += text;
+            polyWidth.push_back(gsl::narrow<int>(cluster.GetColumns()) * coordFontSize.X);
+            cchCharWidths += polyWidth.back();
+            polyWidth.append(text.size() - 1, 0);
         }
 
         // Detect and convert for raster font...
@@ -396,7 +398,7 @@ using namespace Microsoft::Console::Render;
         const auto bottomOffset = _currentLineRendition == LineRendition::DoubleHeightTop ? halfHeight : 0;
 
         pPolyTextLine->lpstr = polyString.data();
-        pPolyTextLine->n = gsl::narrow<UINT>(clusters.size());
+        pPolyTextLine->n = gsl::narrow<UINT>(polyString.size());
         pPolyTextLine->x = ptDraw.x;
         pPolyTextLine->y = ptDraw.y;
         pPolyTextLine->uiFlags = ETO_OPAQUE | ETO_CLIPPED;