Update guiMaterialPreviewWindow.ed.gui

review: revert to full black background for material preview window

Templates/BaseGame/game/tools/materialEditor/gui/guiMaterialPreviewWindow.ed.gui

@@ -174,7 +174,7 @@ $guiContent = new GuiControl() {
          canSave = "1";
          Visible = "1";
          Command = "getColorF($thisControl.color, \"MaterialEditorGui.updatePreviewBackground\");"; 
-         color = "0.04 0.04 0.04 .7";
+         color = "0 0 0 .7";
          hovertime = "1000";
          groupNum = "-1";
          buttonType = "PushButton";