[rbp/omxplayer] Support ff/rew

Big missing feature added for Pi.
It works by increasing the clock and discarding audio when at 2x and 4x.
For rewind and 8x and higher, it uses a different scheme. It pauses video, and single steps.

For both schemes it uses the normal dvd player mechanism of seeking when the clock has diverged by too much.

Notes:
dvdplayer still decodes (then discards) audio when ff/rew.
I'm skipping the decode step (too much cpu at 4x realtime).

I've made GetTime return m_clock.GetClock() when ff/rew. (m_State.time is too jumpy).
Merge pull request #2955 from popcornmix/ff_rew

[rtbp/omxplayer] Support ff/rew