Add getThreadTesselator to the API

src/main/java/com/gtnewhorizon/gtnhlib/api/CapturingTesselator.java

@@ -1,10 +1,23 @@
 package com.gtnewhorizon.gtnhlib.api;
 
 import com.gtnewhorizon.gtnhlib.client.renderer.TessellatorManager;
+import net.minecraft.client.renderer.Tessellator;
 
-public class CapturingTesselator {
-
-    public static boolean isCapturing() {
+@SuppressWarnings("unused")
+public interface CapturingTesselator {
+    /**
+     * @return True if this thread is capturing quads, false otherwise
+     */
+    static boolean isCapturing() {
         return TessellatorManager.isCurrentlyCapturing();
     }
+
+    /**
+     * @throws IllegalStateException If the thread is not capturing and is not the main one.
+     * @return The CapturingTesselator for this thread if capturing, or else {@link Tessellator#instance} if on the main
+     * one.
+     */
+    static Tessellator getThreadTesselator() {
+        return TessellatorManager.get();
+    }
 }