Can you give me the updated code with theses changes

Using OpenGL or VBO for rendering will improve the performance of the code by reducing the number of draw calls and improving the rendering speed.
Using multithreading and batching for rendering will improve the performance of the code by allowing multiple threads to work on different parts of the rendering process simultaneously.

public class UndergroundTexture extends Texture {
	
	private Mw mw;
	private int px = 0;
	private int py = 0;
	private int pz = 0;
	private int dimension = 0;
	private int updateX;
	private int updateZ;
	private byte[][] updateFlags = new byte[9][256];
	private Point[] loadedChunkArray;
	private int textureSize;
	private int textureChunks;
	private int[] pixels;
	
	class RenderChunk implements IChunk {
		Chunk chunk;
		
		public RenderChunk(Chunk chunk) {
			this.chunk = chunk;
		}
		
		@Override
		public int getMaxY() {
			return this.chunk.getTopFilledSegment() + 15;
		}
		
		@Override
		public int getBlockAndMetadata(int x, int y, int z) {
			Block block = this.chunk.getBlock(x, y, z);
            int blockid = Block.blockRegistry.getIDForObject(block);
			int meta = this.chunk.getBlockMetadata(x, y, z);
			return ((blockid & 0xfff) << 4) | (meta & 0xf);
		}

		@Override
		public int getBiome(int x, int z) {
			return (int) this.chunk.getBiomeArray()[(z * 16) + x];
		}

		@Override
		public int getLightValue(int x, int y, int z) {
			return this.chunk.getBlockLightValue(x, y, z, 0);
		}
	}

	public UndergroundTexture(Mw mw, int textureSize, boolean linearScaling) {	
		super(textureSize, textureSize, 0x00000000, GL11.GL_NEAREST, GL11.GL_NEAREST, GL11.GL_REPEAT);
		this.setLinearScaling(false);
		this.textureSize = textureSize;
		this.textureChunks = textureSize >> 4;
		this.loadedChunkArray = new Point[this.textureChunks * this.textureChunks];
		this.pixels = new int[textureSize * textureSize];
		Arrays.fill(this.pixels, 0xff000000);
		this.mw = mw;
	}
	
	public void clear() {
		Arrays.fill(this.pixels, 0xff000000);
		this.updateTexture();
	}
	
	public void clearChunkPixels(int cx, int cz) {
		int tx = (cx << 4) & (this.textureSize - 1);
		int tz = (cz << 4) & (this.textureSize - 1);
		for (int j = 0; j < 16; j++) {
			int offset = ((tz + j) * this.textureSize) + tx;
			Arrays.fill(this.pixels, offset, offset + 16, 0xff000000);
		}
		this.updateTextureArea(tx, tz, 16, 16);
	}

	void renderToTexture(int y) {
		this.setPixelBufPosition(0);
		for (int i = 0; i < this.pixels.length; i++) {
			int colour = this.pixels[i];
			int height = (colour >> 24) & 0xff;
			int alpha = (y >= height) ? 255 - ((y - height) * 8) : 0;
			if (alpha < 0) {
				alpha = 0;
			}
			this.pixelBufPut(((alpha << 24) & 0xff000000) | (colour & 0xffffff));
		}
		this.updateTexture();
	}
	
	public int getLoadedChunkOffset(int cx, int cz) {
		int cxOffset = cx & (this.textureChunks - 1);
		int czOffset = cz & (this.textureChunks - 1);
		return (czOffset * this.textureChunks) + cxOffset;
	}
	
	public void requestView(MapView view) {
		int cxMin = ((int) view.getMinX()) >> 4;
		int czMin = ((int) view.getMinZ()) >> 4;
		int cxMax = ((int) view.getMaxX()) >> 4;
		int czMax = ((int) view.getMaxZ()) >> 4;
		for (int cz = czMin; cz <= czMax; cz++) {
			for (int cx = cxMin; cx <= cxMax; cx++) {
				Point requestedChunk = new Point(cx, cz);
				int offset = this.getLoadedChunkOffset(cx, cz);
				Point currentChunk = this.loadedChunkArray[offset];
				if ((currentChunk == null) || !currentChunk.equals(requestedChunk)) {
					this.clearChunkPixels(cx, cz);
					this.loadedChunkArray[offset] = requestedChunk;
				}
			}
		}
	} 
	
	public boolean isChunkInTexture(int cx, int cz) {
		Point requestedChunk = new Point(cx, cz);
		int offset = this.getLoadedChunkOffset(cx, cz);
		Point chunk = this.loadedChunkArray[offset];
		return (chunk != null) && chunk.equals(requestedChunk);
	}

	public void update() {
		this.clearFlags();

		if (this.dimension != this.mw.playerDimension) {
			this.clear();
			this.dimension = this.mw.playerDimension;
		}
		this.px = this.mw.playerXInt;
		this.py = this.mw.playerYInt;
		this.pz = this.mw.playerZInt;

		this.updateX = (this.px >> 4) - 1;
		this.updateZ = (this.pz >> 4) - 1;

		this.processBlock(
				this.px - (this.updateX << 4),
				this.py,
				this.pz - (this.updateZ << 4)
		);

		int cxMax = this.updateX + 2;
		int czMax = this.updateZ + 2;
		WorldClient world = this.mw.mc.theWorld;
		int flagOffset = 0;
		for (int cz = this.updateZ; cz <= czMax; cz++) {
			for (int cx = this.updateX; cx <= cxMax; cx++) {
				if (this.isChunkInTexture(cx, cz)) {
					Chunk chunk = world.getChunkFromChunkCoords(cx, cz);
					int tx = (cx << 4) & (this.textureSize - 1);
					int tz = (cz << 4) & (this.textureSize - 1);
					int pixelOffset = (tz * this.textureSize) + tx;
					byte[] mask = this.updateFlags[flagOffset];
					ChunkRender.renderUnderground(
							this.mw.blockColours,
							new RenderChunk(chunk),
							this.pixels, pixelOffset, this.textureSize,
							this.py, mask
					);
				}
				flagOffset += 1;
			}
		}

		this.renderToTexture(this.py + 1);
	}

	private void clearFlags() {
		for (byte[] chunkFlags : this.updateFlags) {
			Arrays.fill(chunkFlags, ChunkRender.FLAG_UNPROCESSED);
		}
	}
	
	private void processBlock(int xi, int y, int zi) {
		int x = (this.updateX << 4) + xi;
		int z = (this.updateZ << 4) + zi;
		
		int xDist = this.px - x;
		int zDist = this.pz - z;
		
		if (((xDist * xDist) + (zDist * zDist)) <= 256) {
			if (this.isChunkInTexture(x >> 4, z >> 4)) {
				int chunkOffset = ((zi >> 4) * 3) + (xi >> 4);
				int columnXi = xi & 0xf;
				int columnZi = zi & 0xf;
				int columnOffset = (columnZi << 4) + columnXi;
				byte columnFlag = this.updateFlags[chunkOffset][columnOffset];
				
				if (columnFlag == ChunkRender.FLAG_UNPROCESSED) {
					// if column not yet processed
					WorldClient world = this.mw.mc.theWorld;
					Block block = world.getBlock(x, y, z);
					if ((block == null) || !block.isOpaqueCube()) {
						// if block is not opaque
						this.updateFlags[chunkOffset][columnOffset] = (byte) ChunkRender.FLAG_NON_OPAQUE;
						this.processBlock(xi + 1, y, zi);
						this.processBlock(xi - 1, y, zi);
						this.processBlock(xi, y, zi + 1);
						this.processBlock(xi, y, zi - 1);
					} else {
						// block is opaque
						this.updateFlags[chunkOffset][columnOffset] = (byte) ChunkRender.FLAG_OPAQUE;
					}
				}
			}
		}
	}
}