[dshowvideosink] add I420 support

Submitted by Thomas Löwe

Link to original bug (#628258)

Description

Long time ago i've made a patch to support i420 in dshowvideosink.

After the commit from 07/2010 "Improvements contributed from the Moovida projet" this isn't longer working in the evr mode (vmr still works fine).

Could somebody please look at this or implement i420 format for dshow?

Thanks,
Thomas