android: skip tiles that are already outside of viewport

Change-Id: I65f286797b459f30885560577a4dcb3d1108c0bf

android/experimental/LOAndroid3/src/java/org/libreoffice/LOKitThread.java

@@ -27,11 +27,13 @@ public class LOKitThread extends Thread {
     }
 
     private void tileRequest(ComposedTileLayer composedTileLayer, TileIdentifier tileId) {
-        mLayerClient.beginDrawing();
-        CairoImage image = mTileProvider.createTile(tileId.x, tileId.y, tileId.size, tileId.zoom);
-        SubTile tile = new SubTile(image, tileId);
-        composedTileLayer.addTile(tile);
-        mLayerClient.endDrawing(mViewportMetrics);
+        if (composedTileLayer.isStillValid(tileId)) {
+            mLayerClient.beginDrawing();
+            CairoImage image = mTileProvider.createTile(tileId.x, tileId.y, tileId.size, tileId.zoom);
+            SubTile tile = new SubTile(image, tileId);
+            composedTileLayer.addTile(tile);
+            mLayerClient.endDrawing(mViewportMetrics);
+        }
     }
 
     /** Handle the geometry change + draw. */

android/experimental/LOAndroid3/src/java/org/mozilla/gecko/gfx/ComposedTileLayer.java

@@ -201,4 +201,8 @@ public abstract class ComposedTileLayer extends Layer {
         tile.beginTransaction();
         tiles.add(tile);
     }
+
+    public boolean isStillValid(TileIdentifier tileId) {
+        return RectF.intersects(currentViewport, tileId.getRect()) || currentViewport.contains(tileId.getRect());
+    }
 }
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