Add support for tracking (letter-spacing)

Introduces a Tracking property to TextOptions to allow uniform
adjustment of spacing between characters, measured in em units.

Author: Socolin

src/SixLabors.Fonts/TextLayout.cs

@@ -1088,6 +1088,8 @@ VerticalOrientationType.Rotate or
                     glyphAdvance = glyph.AdvanceHeight;
                 }
 
+                glyphAdvance += options.Tracking * glyph.FontMetrics.UnitsPerEm;
+
                 decomposedAdvances[0] = glyphAdvance;
 
                 if (CodePoint.IsTabulation(codePoint))

src/SixLabors.Fonts/TextOptions.cs

@@ -170,6 +170,13 @@ public float LineSpacing
     /// </summary>
     public KerningMode KerningMode { get; set; }
 
+    /// <summary>
+    /// Gets or sets the tracking (letter-spacing) value.
+    /// Tracking adjusts the spacing between all characters uniformly and is measured in em.
+    /// Positive values increase spacing, negative values decrease spacing, and zero applies no adjustment.
+    /// </summary>
+    public float Tracking { get; set; }
+
     /// <summary>
     /// Gets or sets the positioning mode used for rendering decorations.
     /// </summary>

tests/SixLabors.Fonts.Tests/TextLayoutTests.cs

@@ -967,6 +967,25 @@ public void TrueTypeHinting_CanHintSmallOpenSans(char c, FontRectangle expected)
         Assert.Equal(expected, actual, Comparer);
     }
 
+    [Theory]
+    [InlineData("aaaa", 0, 134.0)]
+    [InlineData("awwa", 0, 162.1)]
+    [InlineData("aaaa", 0.1, 153.3)]
+    [InlineData("awwa", 0.1, 181.4)]
+    [InlineData("aaaa", 1, 326.1)]
+    [InlineData("awwa", 1, 354.1)]
+    public void FontTracking_SpaceCharacters(string text, float tracking, float width)
+    {
+        Font font = new FontCollection().Add(TestFonts.OpenSansFile).CreateFont(64);
+        TextOptions options = new(font)
+        {
+            Tracking = tracking,
+        };
+
+        FontRectangle actual = TextMeasurer.MeasureSize(text, options);
+        Assert.Equal(new FontRectangle(0, 0, width, 35.4f), actual, Comparer);
+    }
+
     [Fact]
     public void CanMeasureTextAdvance()
     {