Show textures when they finish loading before the first palette layer

The explorer only reloaded the map for textures once a layer was already
showing. Now that the Minecraft jar is cached, textures can finish while
the first layer is still loading, which left that layer on map colours
until another layer was picked.

Model used: Claude Opus 5.5

core/src/main/java/com/volmit/iris/core/gui/NoiseExplorerGUI.java

@@ -648,7 +648,8 @@ public final class NoiseExplorerGUI extends JPanel {
         }
         blockTextures = textures;
         paletteInspector.showTextureStatus(null);
-        if (palettePreview != null) {
+        // The first layer may still be loading without textures, so it has to be reloaded even before it shows.
+        if (selectedSource != null) {
             loadSelectedSource();
         }
     }