Move screen-related code to screen.c

The file scrcpy.c contains too many different things in addition to the
main logic, so move the screen code to a separate file, screen.c.

app/meson.build

@@ -11,6 +11,7 @@ src = [
     'src/lockutil.c',
     'src/netutil.c',
     'src/scrcpy.c',
+    'src/screen.c',
     'src/server.c',
     'src/strutil.c',
     'src/tinyxpm.c',