Disable the zippy text drawing algorithm for cells that have combining marks...

Disable the zippy text drawing algorithm for cells that have combining marks or surrogate pairs. CG draws combining marks really badly; core text is slow but much better.

Disable fastpath drawing for box-drawing characters.

This should fix the failing tests.

sources/iTermMutableAttributedStringBuilder.m

@@ -125,7 +125,9 @@
 }
 - (void)appendString:(NSString *)string {
-    if (_zippy) {
+    // Require a string length of 1 to avoid using zippy for combining marks, which core graphics
+    // renders poorly. Zippy still has value for using core graphics for nonascii uncombined characters.
+    if (_zippy && string.length == 1) {
         NSInteger i;
         for (i = 0; i < string.length; i++) {
             unichar c = [string characterAtIndex:i];

sources/iTermTextDrawingHelper.m

@@ -1937,6 +1937,9 @@ static BOOL iTermTextDrawingHelperIsCharacterDrawable(screen_char_t *c,
                     orCharacter:code
                       positions:positions
                          offset:(i - indexRange.location) * _cellSize.width];
+            if (characterAttributes.boxDrawing) {
+                [builder disableFastPath];
+            }
         }
     }
     if (builder.length) {