Revert updatePixels() to set modified flag to true

PImage.modify flag is now used for two purposes and should be split into
two variables:
- internally by renderers to signal that pixels[] changed and need to be
drawn
- externally to signal other renderers that this image changed in some
way and they should update their cache

Reverts 3e5500c457

Fixes #5040

core/src/processing/awt/PGraphicsJava2D.java

@@ -2797,7 +2797,7 @@ public class PGraphicsJava2D extends PGraphics {
     if (pixels != null) {
       getRaster().setDataElements(0, 0, pixelWidth, pixelHeight, pixels);
     }
-    modified = false;
+    modified = true;
   }