Fix tmux -CC attach not showing alternate screen (e.g., vim) correctly

When attaching to a tmux session where a pane is on the alternate screen,
setTmuxState: swaps the primary/alt grid pointers but the read-only state
used for rendering never got the updated content. Mark all cells dirty
after the grid swap and after setAltScreen: writes via memcpy so that the
incremental mergeFrom: sync copies the correct content.

Author: gnachman

sources/VT100ScreenMutableState.m

@@ -5820,6 +5820,7 @@ - (void)setAltScreen:(NSArray<NSData *> *)lines {
             o++;
         } while (o < self.altGrid.size.height && length > 0);
     }
+    [self.altGrid markAllCharsDirty:YES updateTimestamps:NO];
 }
 
 
@@ -5843,6 +5844,10 @@ - (void)setTmuxState:(NSDictionary *)state {
         id<VT100GridReading> temp = self.altGrid;
         self.altGrid = self.primaryGrid;
         self.primaryGrid = temp;
+        // After swapping grid pointers, the read-only state's grids still reference old copies.
+        // Mark all cells dirty to force a full resync during mergeFrom:.
+        [self.primaryGrid markAllCharsDirty:YES updateTimestamps:NO];
+        [self.altGrid markAllCharsDirty:YES updateTimestamps:NO];
     }
 
     NSNumber *altSavedX = state[kStateDictAltSavedCX];