app: implement CustomRenderer option

CustomRenderer disables the construction and binding of a GPU
context to the Window. Combined with ViewEvent and gpu.New, a Gio
client can mix Gio UI and custom rendering.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/internal/wm/window.go

@@ -29,6 +29,7 @@ type Options struct {
 	StatusColor     *color.NRGBA
 	NavigationColor *color.NRGBA
 	Orientation     *Orientation
+	CustomRenderer  bool
 }
 
 type WindowMode uint8