overlays: fix crash when recreating mesh

I refactored this, somehow broke it and did not notice. Thanks to polarite for reporting.

Mesh invalidation happens when you go e.g. into water and the the mesh gets recrated, but never loaded. An assertion is hit (mesh not loaded yet) when attempting to draw it.

Author: Bixilon

src/main/java/de/bixilon/minosoft/gui/rendering/framebuffer/world/overlay/overlays/simple/SimpleOverlay.kt

@@ -50,15 +50,16 @@ abstract class SimpleOverlay(
     }
 
     private fun updateMesh() {
-        if (this.mesh == null || invalid) {
-            mesh?.unload()
-        }
+        this.mesh?.unload()
         this.mesh = createMesh()
+        this.mesh?.load()
         this.invalid = false
     }
 
     override fun draw() {
-        updateMesh()
+        if (invalid) {
+            updateMesh()
+        }
         val mesh = this.mesh ?: return
 
         shader.use()