mixer: don't restore volume with different --softvol-max settings

Changing --softvol-max and then resuming would change the volume level
on resume to something different than the original volume. This is
because the user volume setting is always between 0-100, and 100
corresponds to --softvol-max gain.

Avoid that changing -softvol-max and resuming an older file could lead
to a too loud volume level by refusing to restore if --softvol-max
changed.
demux_playlist: allow recognizing format by mime type

This commit just adds basic support. The following commit will add
actual mime types.
demux_playlist: add some mime types
demux_playlist: don't require header for m3u

Because the http playlist URL I had for testing claimed to be m3u by
file extension and mime type, but didn't have the header.

Note that this actually changes behavior only in the case the format is
detected by mime type. Then p->force will be set before calling the
parser, and the header becomes optional.
Fix bogus copyright text

This file was copied from gl_hwdec_vaglx.c to have a basic skeleton, but
no actual code is left from it.
video/out: separate out code to compute window size

Currently, vo_reconfig() calculates the requested window size and sets
the vo->dwidth/dheight fields _if_ VOCTRL_UPDATE_SCREENINFO is
implemented by the VO or the windowing backend. The window size can be
different from the display size if e.g. the --geometry option is used.
It will also set the vo->dx/dy fields and read vo->xinerama_x/y.

It turned out that this is very backwards and actually requires the
windowing backends to workaround these things. There's also
MPOpts.screenwidth/screenheight, which used to map to actual options,
but is now used only to communicate the screen size to the vo.c code
calculating the window size and position.

Change this by making the window geometry calculations available as
separate functions. This commit doesn't change any VO code yet, and just
emulates the old way using the new functions. VO code will remove its
usage of VOCTRL_UPDATE_SCREENINFO and use the new functions directly.
x11: don't use VOCTRL_UPDATE_SCREENINFO

See previous commit.
vo_sdl: don't use VOCTRL_UPDATE_SCREENINFO
command: rename stream-length to file-size, format file size

Signed-off-by: wm4 <wm4@nowhere>
manpage: adjustments to file-size property