new MAX5476 vidoe output implementation

from vvk / Vitaly Okan implementations, see
http://forum.rcdesign.ru/f123/thread296241-6.html#post4486584
and
https://github.com/vbabich/rush-osd/commit/77f183a5807b1fea6c0a2516fd590aba3e1ce98b
Sumary of changes:
- faster SPI speed (x2)
- interrupt-driven (VSYNC) video buffer update to avoid video artifacts
- auto-increment mode for faster output