Fix column count issues with certain ligature. (#4081)

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## Summary of the Pull Request
This change tries to fix column size calculation when shaping return glyphs that represents multiple characters (e.g. ligature).

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## References

This should fix #696.

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## PR Checklist
* [ ] Closes #xxx
* [X] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [ ] Tests added/passed
* [ ] Requires documentation to be updated
* [ ] 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: #xxx

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## Detailed Description of the Pull Request / Additional comments

Currently, it seems like CustomTextLayout::_CorrectGlyphRun generally assumes that glyphs and characters have a 1:1 mapping relationship - which holds true for most trivial scenarios with basic western scripts, and also many, but unfortunately not all, monospace "programming" fonts with programming ligatures.

This change makes terminal correctly processes glyphs that represents multiple characters, by properly accumulating the column counts of all these characters together (which I believe is more close to what this code originally intended to do).

There are still many issues existing in both CustomTextLayout as well as the TextBuffer, and the correct solution to them will likely demand large-scale changes, at least at the scale of #3578. I wish small changes like this can serve as a stop gap solution while we take our time to work on the long-term right thing.

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## Validation Steps Performed

Builds and runs. Manual testing confirmed that it solves #696 with both LigConsalata and Fixedsys Excelsior.

src/renderer/dx/CustomTextLayout.cpp

@@ -404,8 +404,30 @@ CustomTextLayout::CustomTextLayout(gsl::not_null<IDWriteFactory1*> const factory
             // Offsets is how far to move the origin (in pixels) from where it is
             auto& offset = _glyphOffsets.at(i);
 
-            // Get how many columns we expected the glyph to have and mutiply into pixels.
-            const auto columns = _textClusterColumns.at(i);
+            // Get how many columns we expected the glyph to have and multiply into pixels.
+            UINT16 columns = 0;
+            {
+                // Because of typographic features such as ligatures, it is well possible for a glyph to represent
+                //  multiple code points. Previous calls to IDWriteTextAnalyzer::GetGlyphs stores the mapping
+                //  information between code points and glyphs in _glyphClusters.
+                // To properly allocate the columns for such glyphs, we need to find all characters that this glyph
+                //  is representing and add column counts for all the characters together.
+
+                // Find the range for current glyph run in _glyphClusters.
+                const auto runStartIterator = _glyphClusters.begin() + run.textStart;
+                const auto runEndIterator = _glyphClusters.begin() + run.textStart + run.textLength;
+
+                // Find the range of characters that the current glyph is representing.
+                const auto firstIterator = std::find(runStartIterator, runEndIterator, i - run.glyphStart);
+                const auto lastIterator = std::find(runStartIterator, runEndIterator, i - run.glyphStart + 1);
+
+                // Add all allocated column counts together.
+                for (auto j = firstIterator; j < lastIterator; j++)
+                {
+                    const auto charIndex = std::distance(_glyphClusters.begin(), j);
+                    columns += _textClusterColumns.at(charIndex);
+                }
+            }
             const auto advanceExpected = static_cast<float>(columns * _width);
 
             // If what we expect is bigger than what we have... pad it out.