add isValid() to Bitmap interface

vtm-android/src/org/oscim/android/canvas/AndroidBitmap.java

@@ -25,11 +25,15 @@ import android.opengl.GLUtils;
 
 public class AndroidBitmap implements org.oscim.backend.canvas.Bitmap {
 	final Bitmap mBitmap;
-
 	public AndroidBitmap(InputStream inputStream) {
 		mBitmap = BitmapFactory.decodeStream(inputStream);
 	}
 
+	@Override
+	public boolean isValid(){
+		return mBitmap != null;
+	}
+
 	/**
 	 * @param format ignored always ARGB8888
 	 */

vtm-gdx-android/src/org/oscim/android/canvas/AndroidBitmap.java

@@ -85,4 +85,9 @@ public class AndroidBitmap implements org.oscim.backend.canvas.Bitmap {
 	public void recycle() {
 		mBitmap.recycle();
 	}
+
+	@Override
+	public boolean isValid() {
+		return mBitmap != null;
+	}
 }

vtm-gdx-desktop/src/org/oscim/awt/AwtBitmap.java

@@ -134,4 +134,9 @@ public class AwtBitmap implements Bitmap {
 	@Override
 	public void recycle() {
 	}
+
+	@Override
+	public boolean isValid() {
+		return true;
+	}
 }

vtm-gdx-html/src/org/oscim/gdx/client/GwtBitmap.java

@@ -88,4 +88,9 @@ public class GwtBitmap implements Bitmap {
 		}
 	}
 
+	@Override
+	public boolean isValid() {
+		return true;
+	}
+
 }

vtm/src/org/oscim/backend/canvas/Bitmap.java

@@ -60,4 +60,6 @@ public interface Bitmap {
 	 * @param replace true, when glSubImage2D can be used for upload
 	 */
 	void uploadToTexture(boolean replace);
+
+	boolean isValid();
 }