if a logo is bigger than a video and if positioning results in negative
offsets, vlc crashes in the blend function.

This patch adds a warning, and offset is forced to 0 if negative.

Note that a bigger logo is not a problem per se. (Blend truncates the logo).
Only offsetting it with negative values was a problem.

Also update avcodec encoder and switcher x86 checks.

Pointed-out-by: Michał Trzebiatowski <hippie_1968@hotmail.com>

Signed-off-by: Christoph Miebach <christoph.miebach@web.de>

This matches a similar problem on x264: http://mailman.videolan.org/pipermail/x264-devel/2012-January/009073.html

With minor updates by me.
Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>

Show drift value in debug log

The frame rate is retrieved from the capture parameters nowadays. This
option has no effects at the moment.

It might be nice to reintroduce it so that it can select one rate when
a device supports more than one. But that was never implemented so far.

Close #7240