video: fix some vo_direct3d crashes due to FFmpeg nonsense

For incomprehensible reasons, AV_PIX_FMT_GRAY8 (and some others) have a
palette. This literally makes no sense and this issue has bitten us
before, but it is how it is.

This also caused a crash with vo_direct3d: this mapped a texture as
IMGFMT_Y8 (i.e. AV_PIX_FMT_GRAY8), and when copying this, it tried to
copy the non-existent palette.

Fixes #1113.
video: fix redrawing when pausing after framedrop

When pausing after a frame was just dropped, we're logically at the
dropped frame, and thus should redraw the dropped frame. This was
implemented, but didn't work after unpausing for the second time,
because of a minor logic bug.
manpage: clarify vf_pp docs

Make the note about quoting more prominent.
Add a warning that this filter is useless for new files.
TOOLS: idet: remove extra '$' in $(()) expansion