x11: copy WinID option

For the purpose of making "--wid" setable at any time.
w32: copy WinID option

See previous and next commit.
options: --wid can be set at any time

After being bitten by this, I decided that this mostly unnecessary
requirement sucks.

Allowing this makes it easier to use libmpv, because it can be set after
mpv_initialize(). The latest reasonable time an API user can set this
variable is before actually loading a file.

The previous 2 commits make sure nothing bad can happen if the option is
changed at runtime even if a VO is active. The Cocoa backend should be
fine and doesn't need a change.
vo_opengl: make background color configurable

This mainly affects the black bars that are drawn if the window and
video aspect ratios mismatch.
vo_opengl_cb: simplify reconfigure, render transparent if unconfigured

I think that's expected; mpv shouldn't draw anything while no video is
active. This doesn't blend transparently, though.

Also document the vo_opengl_cb thing.
player: change interaction between revert_seek and ab-loops

The result isn't quite what I imagined, because the A-point is never
marked as a seek point (so you can't jump between A and B), but it's
still slightly better than before.
options: enable --ytdl by default

Let's see who complains...