Hi-DPI: tentative auto DPI detection for Linux

See #6472 #4376

arduino-core/src/processing/app/linux/Platform.java

@@ -25,8 +25,11 @@
 import processing.app.PreferencesData;
 import processing.app.legacy.PConstants;
 
+import java.awt.Font;
 import java.io.File;
 
+import javax.swing.UIManager;
+
 
 /**
  * Used by Base for platform-specific tweaking, for instance finding the
@@ -117,4 +120,19 @@ public void openFolder(File file) throws Exception {
   public String getName() {
     return PConstants.platformNames[PConstants.LINUX];
   }
+
+  private int detectedDpi = -1;
+
+  @Override
+  public int getSystemDPI() {
+    if (detectedDpi != -1)
+      return detectedDpi;
+
+    // we observed that JMenu fonts in java follows the
+    // System DPI settings, so we compare it to the standard
+    // font size (12) to obtain a rough estimate of DPI.
+    Font menuFont = UIManager.getFont("Menu.font");
+    detectedDpi = menuFont.getSize() * 96 / 12;
+    return detectedDpi;
+  }
 }