Fix keyPressed for multiple keys

Java2D and FX2D send multiple PRESSED and only one RELEASE event (at
least on Windows). Therefore we have to keep track of what is pressed
and what not. Most keyboards do not support pressing more than ~10 keys
simultaneously, so this should not cause any performance problems.

Fixes #5049

core/src/processing/core/PApplet.java

@@ -658,7 +658,7 @@ public class PApplet implements PConstants {
    * @see PApplet#keyReleased()
    */
   public boolean keyPressed;
-  int keyPressedCount;
+  List<Long> pressedKeys = new ArrayList<>(6);
 
   /**
    * The last KeyEvent object passed into a mouse function.
@@ -2935,13 +2935,14 @@ public class PApplet implements PConstants {
 
     switch (event.getAction()) {
     case KeyEvent.PRESS:
-      keyPressedCount++;
+      Long hash = ((long) keyCode << Character.SIZE) | key;
+      if (!pressedKeys.contains(hash)) pressedKeys.add(hash);
       keyPressed = true;
       keyPressed(keyEvent);
       break;
     case KeyEvent.RELEASE:
-      keyPressedCount--;
-      keyPressed = (keyPressedCount > 0);
+      pressedKeys.remove(((long) keyCode << Character.SIZE) | key);
+      keyPressed = !pressedKeys.isEmpty();
       keyReleased(keyEvent);
       break;
     case KeyEvent.TYPE:
@@ -3123,7 +3124,10 @@ public class PApplet implements PConstants {
   public void focusGained() { }
 
 
-  public void focusLost() { }
+  public void focusLost() {
+    // TODO: if user overrides this without calling super it's not gonna work
+    pressedKeys.clear();
+  }