screenshots: set AVFrame parameters

Currently, libavcodec is rather lenient, but it might get stricter in
the future.

Fixes #1398.
cocoa: move the gl function loader to gl_cocoa.c
stream_pvr: sort channel list by --tv-channels order

Apparently this is what users would expect.

Going the way of least resistance (in terms of messing with this old,
rarely used code), sorting them by some kind of addition timestamp
(called priority in the patch) is the easiest.

Fixes #1390.
vo_opengl: Ignore approx-gamma for BT.2020-CL

The details of the non-linear transformation from/to BT.2020's constant
luminance system don't really make sense with any other gamma curve,
since changing the gamma curve completely breaks the chroma channels.
w32_common: allow window resizing with --no-border