Prune draw queue before sending to frontend

If the draw queue fills up with lots (i.e. twice the number of max rows)
of insert/delete line commands we can remove draw commands from the
beginning of the draw queue.  This reduces the possibility of the draw
queue overflowing.

src/MacVim/MMBackend.h

@@ -46,6 +46,8 @@
     BOOL                waitForAck;
     int                 initialWindowLayout;
     BOOL                flushDisabled;
+    unsigned            numWholeLineChanges;
+    unsigned            offsetForDrawDataPrune;
 }
 
 + (MMBackend *)sharedInstance;

src/MacVim/MMBackend.m

@@ -82,6 +82,8 @@ - (NSComparisonResult)serverNameCompare:(NSString *)string;
 
 
 @interface MMBackend (Private)
+- (void)clearDrawData;
+- (void)didChangeWholeLine;
 - (void)waitForDialogReturn;
 - (void)insertVimStateMessage;
 - (void)processInputQueue;
@@ -364,7 +366,7 @@ - (void)clearAll
     // Any draw commands in queue are effectively obsolete since this clearAll
     // will negate any effect they have, therefore we may as well clear the
     // draw queue.
-    [drawData setLength:0];
+    [self clearDrawData];
 
     [drawData appendBytes:&type length:sizeof(int)];
 }
@@ -396,6 +398,9 @@ - (void)deleteLinesFromRow:(int)row count:(int)count
     [drawData appendBytes:&bottom length:sizeof(int)];
     [drawData appendBytes:&left length:sizeof(int)];
     [drawData appendBytes:&right length:sizeof(int)];
+
+    if (left == 0 && right == gui.num_cols-1)
+        [self didChangeWholeLine];
 }
 
 - (void)drawString:(char*)s length:(int)len row:(int)row column:(int)col
@@ -431,6 +436,9 @@ - (void)insertLinesFromRow:(int)row count:(int)count
     [drawData appendBytes:&bottom length:sizeof(int)];
     [drawData appendBytes:&left length:sizeof(int)];
     [drawData appendBytes:&right length:sizeof(int)];
+
+    if (left == 0 && right == gui.num_cols-1)
+        [self didChangeWholeLine];
 }
 
 - (void)drawCursorAtRow:(int)row column:(int)col shape:(int)shape
@@ -491,7 +499,7 @@ - (void)flushQueue:(BOOL)force
         [drawData appendBytes:&gui.col length:sizeof(gui.col)];
 
         [self queueMessage:BatchDrawMsgID data:[drawData copy]];
-        [drawData setLength:0];
+        [self clearDrawData];
     }
 
     if ([outputQueue count] > 0) {
@@ -1466,6 +1474,40 @@ - (oneway void)acknowledgeConnection
 
 @implementation MMBackend (Private)
 
+- (void)clearDrawData
+{
+    [drawData setLength:0];
+    numWholeLineChanges = offsetForDrawDataPrune = 0;
+}
+
+- (void)didChangeWholeLine
+{
+    // It may happen that draw queue is filled up with lots of changes that
+    // affect a whole row.  If the number of such changes equals twice the
+    // number of visible rows then we can prune some commands off the queue.
+    //
+    // NOTE: If we don't perform this pruning the draw queue may grow
+    // indefinitely if Vim were to repeatedly send draw commands without ever
+    // waiting for new input (that's when the draw queue is flushed).  The one
+    // instance I know where this can happen is when a command is executed in
+    // the shell (think ":grep" with thousands of matches).
+
+    ++numWholeLineChanges;
+    if (numWholeLineChanges == gui.num_rows) {
+        // Remember the offset to prune up to.
+        offsetForDrawDataPrune = [drawData length];
+    } else if (numWholeLineChanges == 2*gui.num_rows) {
+        // Delete all the unnecessary draw commands.
+        NSMutableData *d = [[NSMutableData alloc]
+                    initWithBytes:[drawData bytes] + offsetForDrawDataPrune
+                           length:[drawData length] - offsetForDrawDataPrune];
+        offsetForDrawDataPrune = [d length];
+        numWholeLineChanges -= gui.num_rows;
+        [drawData release];
+        drawData = d;
+    }
+}
+
 - (void)waitForDialogReturn
 {
     // Keep processing the run loop until a dialog returns.  To avoid getting