Cocoa: Fix string object being updated incorrectly

Fixes #1050.

README.md

@@ -216,6 +216,7 @@ information on what to include when reporting a bug.
 - [Cocoa] Bugfix: Value range was ignored for joystick hats and buttons (#888)
 - [Cocoa] Bugfix: Full screen framebuffer was incorrectly sized for some video
                   modes (#682)
+- [Cocoa] Bugfix: A string object for IME was updated non-idiomatically (#1050)
 - [WGL] Added support for `WGL_EXT_colorspace` for OpenGL ES contexts
 - [WGL] Added support for `WGL_ARB_create_context_no_error`
 - [GLX] Added support for `GLX_ARB_create_context_no_error`

src/cocoa_window.m

@@ -698,10 +698,11 @@ static const NSRange kEmptyRange = { NSNotFound, 0 };
         selectedRange:(NSRange)selectedRange
      replacementRange:(NSRange)replacementRange
 {
+    [markedText release];
     if ([string isKindOfClass:[NSAttributedString class]])
-        [markedText initWithAttributedString:string];
+        markedText = [[NSMutableAttributedString alloc] initWithAttributedString:string];
     else
-        [markedText initWithString:string];
+        markedText = [[NSMutableAttributedString alloc] initWithString:string];
 }
 
 - (void)unmarkText