add method to retrieve single glyph bounds from truetype

this improves performance since we only need to load a single rectangle rather than the entire glyphs array including all points.

src/UglyToad.PdfPig.Fonts/TrueType/Tables/GlyphDataTable.cs

@@ -52,6 +52,39 @@
             glyphs = new Lazy<IReadOnlyList<IGlyphDescription>>(ReadGlyphs);
         }
 
+        public bool TryGetGlyphBounds(int glyphIndex, out PdfRectangle bounds)
+        {
+            bounds = default(PdfRectangle);
+
+            if (glyphIndex < 0 || glyphIndex >= glyphOffsets.Count - 1)
+            {
+                return false;
+            }
+
+            var offset = glyphOffsets[glyphIndex];
+            var nextOffset = glyphOffsets[glyphIndex + 1];
+
+            if (nextOffset <= offset)
+            {
+                bounds = new PdfRectangle(0, 0, 0, 0);
+                return true;
+            }
+            
+            tableBytes.Seek(offset);
+
+            // ReSharper disable once UnusedVariable
+            var contourCount = tableBytes.ReadSignedShort();
+
+            var minX = tableBytes.ReadSignedShort();
+            var minY = tableBytes.ReadSignedShort();
+            var maxX = tableBytes.ReadSignedShort();
+            var maxY = tableBytes.ReadSignedShort();
+
+            bounds = new PdfRectangle(minX, minY, maxX, maxY);
+
+            return true;
+        }
+
         public static GlyphDataTable Load(TrueTypeDataBytes data, TrueTypeHeaderTable table, TableRegister.Builder tableRegister)
         {
             data.Seek(table.Offset);
@@ -81,7 +114,7 @@
 
             for (var i = 0; i < glyphCount; i++)
             {
-                if (offsets[i] == offsets[i + 1])
+                if (offsets[i + 1] <= offsets[i])
                 {
                     // empty glyph
                     result[i] = emptyGlyph;

src/UglyToad.PdfPig.Fonts/TrueType/TrueTypeFont.cs

@@ -113,21 +113,15 @@
                 return false;
             }
             
-            var glyph = TableRegister.GlyphTable.Glyphs[index];
+            if (!TableRegister.GlyphTable.TryGetGlyphBounds(index, out boundingBox))
-
-            if (glyph?.Bounds == null)
             {
                 return false;
             }
 
-            if (glyph.IsEmpty && TryGetBoundingAdvancedWidthByIndex(index, out var advanceWidth))
+            if (boundingBox.Width.Equals(0) && TryGetBoundingAdvancedWidthByIndex(index, out var advanceWidth))
             {
                 boundingBox = new PdfRectangle(0, 0, advanceWidth, 0);
             }
-            else
-            {
-                boundingBox = glyph.Bounds;
-            }
 
             return true;
         }