Memorize getRenderBoundingBox

This takes a non-trivial amount of time on the render thread[^1], so
worth doing.

I don't actually think the allocation is the heavy thing here -
VisualVM says it's toWorldPos being slow. I'm not sure why - possibly
just all the block property lookups? [^2]

[^1]: To be clear, this is with 120 monitors and no other block entities
with custom renderers. so not really representative.

[^2]: I wish I could provide a narrower range, but it varies so much
between me restarting the game. Makes it impossible to benchmark
anything!

src/main/java/dan200/computercraft/shared/peripheral/monitor/TileMonitor.java

@@ -70,6 +70,11 @@ public class TileMonitor extends TileGeneric
     private int xIndex = 0;
     private int yIndex = 0;
 
+    private BlockPos bbPos;
+    private BlockState bbState;
+    private int bbX, bbY, bbWidth, bbHeight;
+    private AxisAlignedBB boundingBox;
+
     public TileMonitor( TileEntityType<? extends TileMonitor> type, boolean advanced )
     {
         super( type );
@@ -624,9 +629,25 @@ public class TileMonitor extends TileGeneric
     @Override
     public AxisAlignedBB getRenderBoundingBox()
     {
+        // We attempt to cache the bounding box to save having to do property lookups (and allocations!) on every frame.
+        // Unfortunately the AABB does depend on quite a lot of state, so we need to add a bunch of extra fields -
+        // ideally these'd be a single object, but I don't think worth doing until Java has value types.
+        if( boundingBox != null && getBlockState().equals( bbState ) && getBlockPos().equals( bbPos ) &&
+            xIndex == bbX && yIndex == bbY && width == bbWidth && height == bbHeight )
+        {
+            return boundingBox;
+        }
+
+        bbState = getBlockState();
+        bbPos = getBlockPos();
+        bbX = xIndex;
+        bbY = yIndex;
+        bbWidth = width;
+        bbHeight = height;
+
         BlockPos startPos = toWorldPos( 0, 0 );
         BlockPos endPos = toWorldPos( width, height );
-        return new AxisAlignedBB(
+        return boundingBox = new AxisAlignedBB(
             Math.min( startPos.getX(), endPos.getX() ),
             Math.min( startPos.getY(), endPos.getY() ),
             Math.min( startPos.getZ(), endPos.getZ() ),