Copy the set of keys from the map before iterating.

To avoid removing elements from the map while iterating over it.
This should solve the Concurrent Modification Exception in the bitmap
garbage collection.

Author: MatanZ

terminal-emulator/src/main/java/com/termux/terminal/TerminalBuffer.java

@@ -616,7 +616,8 @@ public void bitmapGC(int timeDelta) {
                 }
             }
         }
-        for (Integer bn: bitmaps.keySet()) {
+        Set<Integer> keys = new HashSet<Integer>(bitmaps.keySet());
+        for (Integer bn: keys) {
             if (!used.contains(bn)) {
                 bitmaps.remove(bn);
             }