How does one distinguish in Welsh between "eating" and being "eaten"?

In everyday conversation this is the pattern that is used:

The dog is eating the cat - Mae'r ci yn bwyta'r gath
The cat is being eaten by the dog - Mae'r gath yn cael ei bwyta gan y ci

(literally: the cat is getting eaten by the dog)

In formal contexts impersonal short forms of the verb are  used:

Bwyteir y gath gan y ci  The cat is being eaten by the dog 

Bwytawyd y gath gan y ci   The cat was eaten by the dog

These endings -ir and -wyd create versions of the verb which can roughly be translated as:

Bwyteir - there is an eating

Bwytawyd - there was an eating