How to draw into a FBO with proper blending function and save it to a PNG file

BlendingExportDemo.java

import com.badlogic.gdx.Game;
import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.backends.lwjgl.LwjglApplication;
import com.badlogic.gdx.backends.lwjgl.LwjglApplicationConfiguration;
import com.badlogic.gdx.graphics.GL20;
import com.badlogic.gdx.graphics.Pixmap;
import com.badlogic.gdx.graphics.PixmapIO;
import com.badlogic.gdx.graphics.Texture;
import com.badlogic.gdx.graphics.g2d.Batch;
import com.badlogic.gdx.graphics.g2d.SpriteBatch;
import com.badlogic.gdx.utils.ScreenUtils;

public class BlendingExportDemo extends Game{
	public static int w = 1024;
	public static int h = 1024;
	public static void main (String[] arg) {
		LwjglApplicationConfiguration config = new LwjglApplicationConfiguration();
		config.width = w;
		config.height = h;
		new LwjglApplication(new BlendingExportDemo(), config);
	}

	@Override
	public void create() {
	}
	
	@Override
	public void render() {
		Gdx.gl.glClearColor(0, 0, 0, 0);
		Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
		Texture texture = new Texture(Gdx.files.internal("tiled3d/tileset.png"));
		Batch batch = new SpriteBatch();
		batch.getProjectionMatrix().setToOrtho2D(0, 0, Gdx.graphics.getWidth(), Gdx.graphics.getHeight());
		batch.setBlendFunctionSeparate(GL20.GL_SRC_ALPHA, GL20.GL_ONE_MINUS_SRC_ALPHA, GL20.GL_ONE, GL20.GL_ONE);
		batch.begin();
		batch.draw(texture, 0, 0);
		batch.setColor(1, 1, 1, .5f);
		batch.draw(texture, 10, 10);
		batch.end();
		Pixmap pixmap = ScreenUtils.getFrameBufferPixmap(0, 0, w, h);
		PixmapIO.writePNG(Gdx.files.local("test.png"), pixmap);
		pixmap.dispose();
		texture.dispose();
		Gdx.app.exit();
	}
}

note that there is some cases where this technic doesn't properly work. when painting several layers with half transparency, the first layer get mixed with framebuffer background color (black color in this case) which makes the result tinted (darker in this case).
In order to fix it, it's needed to clear the frame buffer with the first layer RGB (Alpha still at zero). This can be done like this :

batch.setBlendFunctionSeparate(GL20.GL_ONE, GL20.GL_ZERO, GL20.GL_ZERO, GL20.GL_ZERO);
// draw the first layer (background)
batch.setBlendFunctionSeparate(GL20.GL_SRC_ALPHA, GL20.GL_ONE_MINUS_SRC_ALPHA, GL20.GL_ONE, GL20.GL_ONE);
// draw all layers (including background layer)