vo_opengl: clamp filters to their size

This gives better results with fancy-downscaling. The issue here is that
fancy-downscalign "extends" the filter radius by some amount, which
requires using a larger filter size and shader. Then most of the filter
is "unused", but some filters still return non-0 coefficients, which
create heavy artifacts. Just clamp them off.

I'm not sure if this is the right solution, but at least it's better
than before.
ytdl: When using DASH, print actual duration

prints the actual duration as reported by youtube-dl to the
terminal when available
ytdl: Try to handle multi-arc videos

this currently uses a sketchy but apparently working workaround,
which will be removed once the neccessary changes in youtube-dl
are implemented

Fixes #1277
ytdl: bump requrired youtube-dl version to 2014.11.26
options: enable --ytdl by default

Let's see who complains...
Release 0.7.2