feat(renderer): add z-index support for fine-grained layer ordering in rendering

src/main/java/de/extio/game_engine/renderer/g2d/G2DRenderer.java

@@ -43,6 +43,8 @@ public class G2DRenderer implements Renderer {
 	private static final Logger LOGGER = LoggerFactory.getLogger(G2DRenderer.class);
 	
 	private static final long SLEEP_PRECISION = TimeUnit.MILLISECONDS.toNanos(2);
+
+	private static final short ZINDEX_STEPS = RenderingBoLayer.UI_TOP - RenderingBoLayer.UI_BGR + 1;
 	
 	private final RingBuffer<Integer> fpsHistory = new RingBuffer<>(90);
 	
@@ -175,7 +177,7 @@ public class G2DRenderer implements Renderer {
 				this.drawStatistics();
 				this.rendererData.getRendererWorkingSet().commit(this.rendererModuleId, true);
 				this.rendererData.getRendererWorkingSet().getLiveSet(this.renderingBOs, this.rendererData.getModuleService()::isDisplayed);
-				this.renderingBOs.sort((bo0, bo1) -> Short.compare(bo0.getLayer(), bo1.getLayer()));
+				this.renderingBOs.sort((bo0, bo1) -> Integer.compare(this.getEffectiveLayer(bo0), this.getEffectiveLayer(bo1)));
 				
 				Graphics2D screenGraphics = null;
 				try {
@@ -204,7 +206,7 @@ public class G2DRenderer implements Renderer {
 					for (final RenderingBo bo : usedRenderingBoTypes.values()) {
 						bo.staticCleanupAfterFrame();
 					}
-
+					
 					this.rendererData.getRenderingBoPool().releasePending();
 					
 					//
@@ -329,6 +331,14 @@ public class G2DRenderer implements Renderer {
 		}
 	}
 	
+	private int getEffectiveLayer(final RenderingBo bo) {
+		final var layer = bo.getLayer();
+		if (layer >= RenderingBoLayer.UI_BGR && layer <= RenderingBoLayer.UI_TOP) {
+			return layer + (bo.getZIndex() * ZINDEX_STEPS);
+		}
+		return layer;
+	}
+	
 	public G2DMainFrame getMainFrame() {
 		return this.mainFrame;
 	}

src/main/java/de/extio/game_engine/renderer/model/AbstractRenderingBo.java

@@ -33,6 +33,8 @@ public abstract class AbstractRenderingBo implements RenderingBo {
 	
 	private final short defaultLayer;
 	
+	protected short zIndex;
+	
 	public AbstractRenderingBo(final short layer) {
 		this.layer = layer;
 		this.defaultLayer = layer;
@@ -224,6 +226,7 @@ public abstract class AbstractRenderingBo implements RenderingBo {
 		this.width = 0;
 		this.height = 0;
 		this.layer = this.defaultLayer;
+		this.zIndex = 0;
 	}
 
 	@Override
@@ -238,6 +241,7 @@ public abstract class AbstractRenderingBo implements RenderingBo {
 			this.width = abstractRenderingBo.width;
 			this.height = abstractRenderingBo.height;
 			this.layer = abstractRenderingBo.layer;
+			this.zIndex = abstractRenderingBo.zIndex;
 		}
 	}
 	
@@ -298,6 +302,17 @@ public abstract class AbstractRenderingBo implements RenderingBo {
 		return this;
 	}
 	
+	@Override
+	public short getZIndex() {
+		return this.zIndex;
+	}
+	
+	@Override
+	public RenderingBo setZIndex(final short zIndex) {
+		this.zIndex = zIndex;
+		return this;
+	}
+	
 	public int getWidth() {
 		return this.width;
 	}

src/main/java/de/extio/game_engine/renderer/model/RenderingBo.java

@@ -35,6 +35,16 @@ public interface RenderingBo extends AutoCloseable {
 	
 	short getLayer();
 	
+	/**
+	 * Gets the z-index for fine-grained ordering within UI layers (UI_BGR to UI_TOP).
+	 */
+	short getZIndex();
+	
+	/**
+	 * Sets the z-index for fine-grained ordering within UI layers (UI_BGR to UI_TOP).
+	 */
+	RenderingBo setZIndex(short zIndex);
+	
 	/**
 	 * Positions this bo with an absolute position on the viewport. x0 y0 coordinates are at the top left. Call RendererControl#getEffectiveViewportDimension to get the absolute dimension of the viewport.
 	 */

src/test/java/de/extio/game_engine/renderer/work/RendererWorkingSetImplTest.java

@@ -426,6 +426,16 @@ public class RendererWorkingSetImplTest {
 		public short getLayer() {
 			return RenderingBoLayer.UI0;
 		}
+
+		@Override
+		public short getZIndex() {
+			return 0;
+		}
+
+		@Override
+		public RenderingBo setZIndex(short zIndex) {
+			return this;
+		}
 		
 		@Override
 		public RenderingBo withPositionAbsoluteAnchorTopLeft(final int x, final int y) {

src/test/java/de/extio/game_engine/renderer/work/RenderingBoPoolImplTest.java

@@ -343,6 +343,16 @@ public class RenderingBoPoolImplTest {
 			return 0;
 		}
 
+		@Override
+		public short getZIndex() {
+			return 0;
+		}
+
+		@Override
+		public RenderingBo setZIndex(short zIndex) {
+			return this;
+		}
+
 		@Override
 		public RenderingBo withPositionAbsoluteAnchorTopLeft(final int x, final int y) {
 			return this;
@@ -492,6 +502,16 @@ public class RenderingBoPoolImplTest {
 			return 0;
 		}
 
+		@Override
+		public short getZIndex() {
+			return 0;
+		}
+
+		@Override
+		public RenderingBo setZIndex(short zIndex) {
+			return this;
+		}
+		
 		@Override
 		public RenderingBo withPositionAbsoluteAnchorTopLeft(final int x, final int y) {
 			return this;