src/output.c: Always react to new output configuration

src/output.c: Always react to new output configuration

Without this fix we will never react to output configuration changes
via wlr-randr and friends.

If output_config_apply() commits the new config, handle_output_layout_change()
is called but doesn't do anything because the config is still pending.

This patch moves the code into its own function do_output_layout_change()
which additionally gets called after all output configs are committed.

The original handler is turned into a wrapper around do_output_layout_change().

Reported-by @heroin-moose