revid/revid.go: create constants for MOG filter arguments

[Saxon Milton]

On initialisation of our MOG filer:

‌

r.filter = filter.NewMOGFilter(r.encoders, 25, 20, 500, 3, true)

We should be using constants for the integer arguments.

[Scott Barnard]

Assigned to me @Scott Barnard

[Saxon Milton]

Resolved by PR #318