fix #124 insensitive tap to drag on low dpi screens

app/src/main/java/com/openathena/MarkableImageView.java

@@ -11,6 +11,7 @@
 import android.graphics.Paint;
 import android.net.Uri;
 import android.util.AttributeSet;
+import android.util.DisplayMetrics;
 import android.util.Log;
 import android.view.GestureDetector;
 import android.view.MotionEvent;
@@ -49,6 +50,10 @@ public MarkableImageView(Context context, AttributeSet attrs) {
         }
         parent = (AthenaActivity) context;
 
+        DisplayMetrics metrics = new DisplayMetrics();
+        float screen_pixel_density = getResources().getDisplayMetrics().density;
+        final float clickThreshold = 15f * screen_pixel_density;
+
         scaleGestureDetector = new ScaleGestureDetector(context, new MyScaleGestureListener());
         gestureDetector = new GestureDetector(context, new GestureDetector.SimpleOnGestureListener() {
             @Override
@@ -73,7 +78,6 @@ public boolean onDoubleTap(MotionEvent e) {
         MarkableImageView yahweh = this; // reference to this MarkableImageView, for use in listener
 
         this.setOnTouchListener(new View.OnTouchListener() {
-            private final float clickThreshold = 25f;
             private float lastX, lastY;