Correctly calculate buffer positions in vertex buffer writer

This should fix certain memory corruption-related crashes, e.g. with Mekanism
pipes.

Author: FiniteReality

src/main/java/org/embeddedt/embeddium/impl/mixin/core/render/immediate/consumer/BufferBuilderMixin.java

@@ -25,12 +25,17 @@ public abstract class BufferBuilderMixin implements VertexBufferWriter {
 
     @Shadow
     private int vertices;
+
+    @Shadow
+    private int elementsToFill;
+
     @Shadow
     @Final
     private int vertexSize;
 
     @Shadow
     private long vertexPointer;
+
     @Unique
     private VertexFormatDescription embeddiumFormat;
 
@@ -61,8 +66,8 @@ public void push(MemoryStack stack, long src, int count, VertexFormatDescription
         }
 
         this.vertices += count;
-        // TODO - probably not needed?
-        this.vertexPointer = dst;
+        this.vertexPointer = dst + length - this.vertexSize;
+        this.elementsToFill = 0;
     }
 
     @Unique