app/internal/window: run main on main thread on Android and iOS

Before this change, Android and iOS were special for two reasons:
app.Main would return immediately, and the program main was invoked from
a goroutine. We can reduce that to one special case by either

- run the program main from the main thread,
- or make app.Main block.

Choose to run main on the main thread.

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

app/internal/window/runmain.go

@@ -24,6 +24,6 @@ func runMain() {
 		// Indirect call, since the linker does not know the address of main when
 		// laying down this package.
 		fn := mainMain
-		go fn()
+		fn()
 	})
 }