Clear render targets upon creation to avoid having them contain undef…

…ined data

We interpreted the buffer clear directives a bit too literally. It's always necessary to clear color buffers before rendering to them.

src/main/java/net/coderbot/iris/rendertarget/RenderTargets.java

@@ -6,13 +6,16 @@
 import java.util.Map;
 
 import com.google.common.collect.ImmutableSet;
+import com.mojang.blaze3d.platform.GlStateManager;
+import com.mojang.blaze3d.systems.RenderSystem;
 import net.coderbot.iris.Iris;
 import net.coderbot.iris.gl.framebuffer.GlFramebuffer;
-import net.coderbot.iris.gl.texture.InternalTextureFormat;
-import net.coderbot.iris.shaderpack.PackDirectives;
 
 import net.coderbot.iris.shaderpack.PackRenderTargetDirectives;
+import net.minecraft.client.MinecraftClient;
 import net.minecraft.client.gl.Framebuffer;
+import org.lwjgl.opengl.GL20C;
+import org.lwjgl.opengl.GL30C;
 
 public class RenderTargets {
 	/**
@@ -49,6 +52,21 @@ public RenderTargets(int width, int height, Map<Integer, PackRenderTargetDirecti
 		this.cachedHeight = height;
 
 		this.ownedFramebuffers = new ArrayList<>();
+
+		// NB: Make sure all buffers are cleared so that they don't contain undefined
+		// data. Otherwise very weird things can happen.
+		//
+		// TODO: Make this respect the clear color of each buffer, destroy these framebuffers afterwards.
+		RenderSystem.clearColor(0.0f, 0.0f, 0.0f, 0.0f);
+
+		createFramebufferWritingToMain(new int[] {0,1,2,3,4,5,6,7}).bind();
+		RenderSystem.clear(GL20C.GL_COLOR_BUFFER_BIT, false);
+
+		createFramebufferWritingToAlt(new int[] {0,1,2,3,4,5,6,7}).bind();
+		RenderSystem.clear(GL20C.GL_COLOR_BUFFER_BIT, false);
+
+		// Make sure to rebind the vanilla framebuffer.
+		MinecraftClient.getInstance().getFramebuffer().beginWrite(false);
 	}
 
 	public void destroy() {