weston-scaler: add transform option

Like simple-damage add an transform option. Currently there is no
other test which covers combinations like MODE_SRC_ONLY with a
transform of 90, which are easy to get wrong.
1 job from scaler-add-transforms in 4 minutes and 1 second (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Build
passed #42878
build-native

00:04:01