Add an advanced pref to allow fast but poor quality combining mark rendering.

sources/iTermAdvancedSettingsModel.h

@@ -191,5 +191,6 @@
 + (NSString *)viewManPageCommand;
 + (BOOL)preventEscapeSequenceFromClearingHistory;
 + (BOOL)dwcLineCache;
++ (BOOL)lowFiCombiningMarks;
 @end

sources/iTermAdvancedSettingsModel.m

@@ -157,6 +157,8 @@ DEFINE_FLOAT(pointSizeOfTimeStamp, 10, @"General: Point size for timestamps");
 DEFINE_INT(terminalMargin, 5, @"General: Width of left and right margins in terminal panes\nHow much space to leave between the left and right edges of the terminal.\nYou must restart iTerm2 after modifying this property. Saved window arrangements should be re-created.");
 DEFINE_INT(terminalVMargin, 2, @"General: Height of top and bottom margins in terminal panes\nHow much space to leave between the top and bottom edges of the terminal.\nYou must restart iTerm2 after modifying this property. Saved window arrangements should be re-created.");
 DEFINE_BOOL(zippyTextDrawing, YES, @"General: Use zippy text drawing algorithm?\nThis draws non-ASCII text more quickly but with lower fidelity. This setting is ignored if ligatures are enabled in Prefs > Profiles > Text.");
+DEFINE_BOOL(lowFiCombiningMarks, NO, @"General: Prefer speed to accuracy for characters with combining marks?");
+
 DEFINE_BOOL(useVirtualKeyCodesForDetectingDigits, YES, @"General: On keyboards that require a modifier to press a digit, do not require that modifier for switching between windows, tabs, and panes by number.\nFor example, AZERTY requires you to hold down Shift to enter a number. To switch tabs with Cmd+Number on an AZERTY keyboard, you must enable this setting. Then, for example, Cmd-& switches to tab 1. When this setting is enabled, some user-defined shortcuts may become unavailable because the tab/window/pane switching behavior takes precedence.");
 DEFINE_STRING(viewManPageCommand, @"man %@ || sleep 3", @"General: Command to view man pages.\nUsed when you press the man page button on the touch bar. %@ is replaced with the command. End the command with & to avoid opening an iTerm2 window (e.g., if you're launching an external viewer).");

sources/iTermMutableAttributedStringBuilder.m

@@ -127,7 +127,11 @@
 - (void)appendString:(NSString *)string {
     // 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) {
+    BOOL tryZippy = _zippy;
+    if (tryZippy && ![iTermAdvancedSettingsModel lowFiCombiningMarks] && string.length > 1) {
+        tryZippy = NO;
+    }
+    if (tryZippy) {
         NSInteger i;
         for (i = 0; i < string.length; i++) {
             unichar c = [string characterAtIndex:i];