app: confine the eglWindow indirection to the Wayland backend

Only the Wayland backend needs an wl_egl_window between the wl_surface
and EGL. Move code dealing with the indirection to Wayland specific
code.

Then, introduce the eglDriver interface instead of referencing the
native window type directly. This will help when multiple backends are
supported at runtime (e.g. Wayland+X11).

Finally, move the eglDriver implementation methods from GOOS-specific
code to separate EGL-specific files, allowing EGL types to be used
directly instead of unsafe.Pointer and uinptr.

The result is simpler generic EGL code, and easier path towards X11
support.

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

app/os_android.go

@@ -245,11 +245,7 @@ func (w *window) setStage(stage Stage) {
 	w.event(StageEvent{stage})
 }
 
-func (w *window) display() unsafe.Pointer {
-	return nil
-}
-
-func (w *window) nativeWindow(visID int) (unsafe.Pointer, int, int) {
+func (w *window) nativeWindow(visID int) (*C.ANativeWindow, int, int) {
 	win := w.aNativeWindow()
 	var width, height int
 	if win != nil {
@@ -259,7 +255,7 @@ func (w *window) nativeWindow(visID int) (unsafe.Pointer, int, int) {
 		w, h := C.ANativeWindow_getWidth(win), C.ANativeWindow_getHeight(win)
 		width, height = int(w), int(h)
 	}
-	return unsafe.Pointer(win), width, height
+	return win, width, height
 }
 
 func (w *window) aNativeWindow() *C.ANativeWindow {