add a different way of running open cover since it doesn't work on app veyor

src/CodeCoverage/CodeCoverage.cmd

@@ -0,0 +1,15 @@
+@echo off
+
+cd src\CodeCoverage
+
+nuget restore packages.config -PackagesDirectory .
+
+cd ..
+
+dotnet restore  UglyToad.Pdf.sln
+dotnet build  UglyToad.Pdf.sln --no-incremental -c debug /p:codecov=true
+ 
+rem The -threshold options prevents this taking ages...
+CodeCoverage\OpenCover.4.6.519\tools\OpenCover.Console.exe -target:"dotnet.exe" -targetargs:"test UglyToad.Pdf.Tests\UglyToad.Pdf.Tests.csproj --no-build -c debug" -register:user -output:.\test-results.xml -hideskipped:All -returntargetcode -oldStyle -filter:"+[UglyToad.Pdf*]* -[UglyToad.Pdf.Tests*]*" 
+
+if %errorlevel% neq 0 exit /b %errorlevel%

src/CodeCoverage/packages.config

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+<?xml version="1.0" encoding="utf-8"?>
+<packages>
+  <package id="OpenCover" version="4.6.519" />
+</packages>

src/UglyToad.Pdf/Fonts/TrueType/Tables/IndexToLocationTable.cs

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+namespace UglyToad.Pdf.Fonts.TrueType.Tables
+{
+    /// <summary>
+    /// Stores the offset to the glyph locations relative to the start of the glyph data table.
+    /// Index zero points to the "missing character" which is used for characters not provided by the font.
+    /// The number of glpyhs in this table should match the maximum profile table.
+    /// </summary>
+    internal class IndexToLocationTable : ITable
+    {
+        public string Tag => TrueTypeHeaderTable.Loca;
+
+        public TrueTypeHeaderTable DirectoryTable { get; }
+
+        public long[] GlyphOffsets { get; }
+
+        public IndexToLocationTable(TrueTypeHeaderTable directoryTable, long[] glyphOffsets)
+        {
+            DirectoryTable = directoryTable;
+            GlyphOffsets = glyphOffsets;
+        }
+
+        public static IndexToLocationTable Load(TrueTypeDataBytes data, TrueTypeHeaderTable table)
+        {
+            data.Seek(table.Offset);
+
+            return null;
+        }
+    }
+}