Draw the specified background if present

Author: Miha-x64

android-pdf-viewer/src/main/java/com/github/barteksc/pdfviewer/PDFView.java

@@ -494,7 +494,12 @@ protected void onDraw(Canvas canvas) {
         // abstraction of the screen position when rendering the parts.
 
         // Draws background
-        canvas.drawColor(Color.WHITE);
+        Drawable bg = getBackground();
+        if (bg == null) {
+            canvas.drawColor(Color.WHITE);
+        } else {
+            bg.draw(canvas);
+        }
 
         if(recycled) {
             return;