compositor: Fix default transforms when output section declared

compositor: Fix default transforms when output section declared

Regardless of the default transform passed in, weston_parse_transform
would always return 'normal' if there was an output section. This is
because, if a section was declared for that output, it would ask
weston_config for the transform, with the default being 'normal'.

Fix it so we return the passed-in default transform when we have a
matching output section without a transform key. If the transform is
declared but invalid, we can remove the line resetting to the default
transform, because we've already set the default transform up top.

Signed-off-by: Daniel Stone <daniels@collabora.com>