TerrainQuad: serialize "patchSize" field.

[Ali-RS]

After loading a terrain from j3o, calling TerrainQuad.getPatchSize() will return 0, because "patchSize" is ignored when writing to file. This patch should fix it.

[stephengold]

Looks good. Two requests:

1. Update the copyright date to include 2021.
2. Add a test case. You can use TestCloneMesh.java as a starting point.

[Ali-RS]

@stephengold thanks for reviewing.

Is this fine for the test?

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package com.jme3.terrain;

import com.jme3.export.binary.BinaryExporter;
import com.jme3.math.FastMath;
import com.jme3.terrain.collision.BaseAWTTest;
import com.jme3.terrain.geomipmap.TerrainQuad;
import com.jme3.terrain.heightmap.AbstractHeightMap;
import com.jme3.terrain.heightmap.ImageBasedHeightMap;
import com.jme3.texture.Texture;
import org.junit.Assert;
import org.junit.Test;

/**
 * Test saving/loading terrain.
 *
 * @author Ali-RS
 */
public class TestTerrainExporting extends BaseAWTTest {
    
    /**
     * Test saving/loading a TerrainQuad.
     */
    @Test
    public void testTerrainExporting() {

        Texture heightMapImage = getAssetManager().loadTexture("Textures/Terrain/splat/mountains512.png");
        AbstractHeightMap map = new ImageBasedHeightMap(heightMapImage.getImage(), 0.25f);
        map.load();

        TerrainQuad terrain = new TerrainQuad("Terrain", 65, 513, map.getHeightMap());

        TerrainQuad saveAndLoad = BinaryExporter.saveAndLoad(getAssetManager(), terrain);

        Assert.assertEquals(513, saveAndLoad.getTotalSize());
        Assert.assertEquals(65, saveAndLoad.getPatchSize());
        Assert.assertArrayEquals(terrain.getHeightMap(), saveAndLoad.getHeightMap(), FastMath.ZERO_TOLERANCE);
    }

}

[stephengold]

That's sufficient. Thank you.

[Ali-RS]

Committed the new changes as requested.