Yet another optimization

src/FontStashSharp/Font.cs

@@ -6,10 +6,10 @@ namespace FontStashSharp
 {
 	internal unsafe class Font: IDisposable
 	{
-		private readonly byte[] Data;
 		private GCHandle? dataPtr = null;
 		private float AscentBase, DescentBase, LineHeightBase;
-
+		readonly Int32Map<int> _kernings = new Int32Map<int>();
+
 		public float Ascent { get; private set; }
 		public float Descent { get; private set; }
 		public float LineHeight { get; private set; }
@@ -24,8 +24,6 @@ public Font(byte[] data)
 				throw new ArgumentNullException(nameof(data));
 			}
 
-			Data = data;
-
 			dataPtr = GCHandle.Alloc(data, GCHandleType.Pinned);
 		}
 
@@ -76,6 +74,19 @@ public void RenderGlyphBitmap(byte *output, int outWidth, int outHeight, int out
 			stbtt_MakeGlyphBitmap(_font, output, outWidth, outHeight, outStride, Scale, Scale, glyph);
 		}
 
+		public int GetGlyphKernAdvance(int glyph1, int glyph2)
+		{
+			var key = ((glyph1 << 16) | (glyph1 >> 16)) ^ glyph2;
+			int result;
+			if (_kernings.TryGetValue(key, out result))
+			{
+				return result;
+			}
+			result = stbtt_GetGlyphKernAdvance(_font, glyph1, glyph2);
+			_kernings[key] = result;
+			return result;
+		}
+
 		public static Font FromMemory(byte[] data)
 		{
 			var font = new Font(data);

src/FontStashSharp/FontGlyph.cs

@@ -4,8 +4,6 @@ namespace FontStashSharp
 {
 	class FontGlyph
 	{
-		private readonly Int32Map<int> _kernings = new Int32Map<int>();
-
 		public Font Font;
 		public FontAtlas Atlas;
 		public int Codepoint;
@@ -28,18 +26,5 @@ public bool IsEmpty
 				return Bounds.Width == 0 || Bounds.Height == 0;
 			}
 		}
-
-		public int GetKerning(FontGlyph nextGlyph)
-		{
-			int result;
-			if (_kernings.TryGetValue(nextGlyph.Index, out result))
-			{
-				return result;
-			}
-			result = StbTrueTypeSharp.StbTrueType.stbtt_GetGlyphKernAdvance(Font._font, Index, nextGlyph.Index);
-			_kernings[nextGlyph.Index] = result;
-
-			return result;
-		}
 	}
 }

src/FontStashSharp/FontSystem.cs

@@ -825,7 +825,7 @@ private void GetQuad(FontGlyph glyph, FontGlyph prevGlyph, float spacing, ref fl
 				float adv = 0;
 				if (UseKernings && glyph.Font == prevGlyph.Font)
 				{
-					adv = prevGlyph.GetKerning(glyph) * glyph.Font.Scale;
+					adv = prevGlyph.Font.GetGlyphKernAdvance(prevGlyph.Index, glyph.Index) * glyph.Font.Scale;
 				}
 
 				x += (int)(adv + spacing + 0.5f);