cache bounding boxes for composite fonts

cached the bounding box for a specific character code value for type 0 (composite) fonts to improve performance.

src/UglyToad.PdfPig/PdfFonts/Composite/Type0Font.cs

@@ -1,6 +1,7 @@
 namespace UglyToad.PdfPig.PdfFonts.Composite
 {
     using System;
+    using System.Collections.Generic;
     using CidFonts;
     using Cmap;
     using Core;
@@ -16,6 +17,8 @@
         private readonly CMap ucs2CMap;
         // ReSharper disable once NotAccessedField.Local
         private readonly bool isChineseJapaneseOrKorean;
+        private readonly Dictionary<int, CharacterBoundingBox> boundingBoxCache
+            = new Dictionary<int, CharacterBoundingBox>();
 
         public NameToken Name => BaseFont;
 
@@ -84,6 +87,11 @@
 
         public CharacterBoundingBox GetBoundingBox(int characterCode)
         {
+            if (boundingBoxCache.TryGetValue(characterCode, out var cached))
+            {
+                return cached;
+            }
+
             var matrix = GetFontMatrix();
 
             var boundingBox = GetBoundingBoxInGlyphSpace(characterCode);
@@ -96,7 +104,11 @@
 
             var advanceWidth = matrix.TransformX(width);
 
-            return new CharacterBoundingBox(boundingBox, advanceWidth);
+            var result = new CharacterBoundingBox(boundingBox, advanceWidth);
+
+            boundingBoxCache[characterCode] = result;
+
+            return result;
         }
 
         public PdfRectangle GetBoundingBoxInGlyphSpace(int characterCode)