m_option: make defval const

There's really no reason why it shouldn't.
m_option: add missing copy callback for some option types

One reaosn for this is that this broke proper handling of .defval, since
m_option_copy did nothing for these option types.
vo_vdpau: allow setting colorkey

Formally, this sets the "background color" of the presentation queue.
But in practice, this color is also used as colorkey.

This commit doesn't change the VDPAU default yet.
vo_vdpau: use color close to black as default colorkey (instead of green)

The VDPAU default colorkey, although it seems to be driver specific, is
usually green. This is a pretty annoying color, and you usually see it
briefly (as flashes) if the VDPAU window resizes.

Change it to some shade of black. The new default color is close to what
MPlayer picks as colorkey (and apparently it worked well for them):

    VdpColor vdp_bg = {0.01, 0.02, 0.03, 0};

Since our OPT_COLOR can set 8 bit colors only, we use '#020507' instead,
which should be the same assuming 8 bit colors.

Obviously, you can't use black, because black is a way too common color,
and would make it too easy to observe the colorkey effect when e.g.
moving a terminal with black background over the video window.