First pass at a stateful renderer.

This isn't done: Other code paths are hacked, out and this commit
message isn't nearly descriptive enough.

Author: s4y

src/MacVim/MMCoreTextView.h

@@ -40,6 +40,7 @@
     CGGlyph                     *glyphs;
     CGPoint                     *positions;
     NSMutableArray              *fontCache;
+    NSMutableDictionary<NSNumber*,NSFont*> *fontTraitCache;
 
     // Issue draws onto an CGImage that caches the drawn results instead of
     // directly in drawRect:. This is the default behavior in cases where simply