[Patch] Add scrollwheel to cocoa drawterm

[Steven McDonald <steven@steven-mcdonald.id.au>]

This should match the behaviour of gui-osx, and it works for me. As far
as I can tell, this can't just be done with the existing mouseevent
function because deltaY is used to indicate both mouse movement and
scrollwheel movement, so the function needs to know for sure that it's
dealing with a scrollwheel event.

Thanks for the work on this, aiju!

diff -r 56cee9a36593 gui-cocoa/screen.m
--- a/gui-cocoa/screen.m	Tue Jul 03 23:28:16 2018 +0200
+++ b/gui-cocoa/screen.m	Thu Jul 05 20:13:55 2018 +0200
@@ -212,6 +212,7 @@
 - (void) otherMouseDown:(NSEvent*)event;
 - (void) otherMouseDragged:(NSEvent*)event;
 - (void) otherMouseUp:(NSEvent*)event;
+- (void) scrollWheel:(NSEvent*)event;
 - (BOOL) acceptsFirstResponder;
 - (void) reshape;
 - (BOOL) acceptsMouseMovedEvents;
@@ -406,6 +407,10 @@
 - (void) otherMouseDragged:(NSEvent*)event { [self mouseevent:event]; }
 - (void) otherMouseUp:(NSEvent*)event { [self mouseevent:event]; }
 
+- (void) scrollWheel:(NSEvent*)event {
+	mousetrack(0, 0, [event deltaY]>0 ? 8 : 16, ticks());
+}
+
 - (BOOL) acceptsFirstResponder {
 	return TRUE;
 }