Canceled participants count toward registration cap

1.0 Problem/Motivation

When webform_civicrm is deciding which events are full, it counts all registrations, even those that shouldn't be, such as canceled registrations.

2.0 Detailed steps to reproduce (embed screenshots)

* Create an event.
* Set its max capacity to 1.
* Create a webform that allows users to select the event they want to register for, using parameters that would include this event.
* Ensure that registering for full events is blocked.
* Visit the form, note that the event is visible.
* Register someone for the event, note that the event is no longer visible.
* Change the participant status on that registration to "Canceled". Reload the form - the event is still not visible.

3.0 Proposed resolution
I have a PR I'm going to submit.