V4L2 padding removal, and support for H264_I_PERIOD

Needs my firmware changes to be picked up too (C/Ls 451759, 451760, and 451774) to provide full functionality, otherwise some image formats with width/height not a multiple of 32/16 will fail in colourful ways.

First change is to address https://github.com/raspberrypi/linux/issues/593