Don't use rounded corners for PopupMenus in the editor theme

PopupMenus use separate windows by default in the editor, and windows
no longer support per-pixel transparency for now in 4.0.
Even if per-pixel transparency was reimplemented, we can't assume
that it'll always work in practice (e.g. when compositing is disabled).

editor/editor_themes.cpp

@@ -912,6 +912,9 @@ Ref<Theme> create_editor_theme(const Ref<Theme> p_theme) {
 	// Always display a border for PopupMenus so they can be distinguished from their background.
 	style_popup_menu->set_border_width_all(EDSCALE);
 	style_popup_menu->set_border_color(dark_color_2);
+	// Popups are separate windows by default in the editor. Windows currently don't support per-pixel transparency
+	// in 4.0, and even if it was, it may not always work in practice (e.g. running with compositing disabled).
+	style_popup_menu->set_corner_radius_all(0);
 	theme->set_stylebox("panel", "PopupMenu", style_popup_menu);
 
 	Ref<StyleBoxFlat> style_menu_hover = style_widget_hover->duplicate();