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
chmap: add a 7.1(rear) layout name

This is common on Apple systems so it's handy to have a label for it.
chmap_sel: add multichannel fallback heuristic

Instead of just failing during channel map selection, try to select a close
layout that makes most sense and upmix/downmix to that instead of failing AO
initialization. The heuristic is rather simple, and uses the following steps:

1) If mono is required always prefer stereo to a multichannel upmix.
2) Search for an upmix that is an exact superset of the required channel map.
3) Search for a downmix that is the exact subset of the required channel map.
4) Search for either an upmix or downmix that is the closest (minimum difference
   of channels) to the required channel map.
ao: add debug log with the detected channel maps

This could be helpful with bug reports.
rpi: initial commit

Setup the audio decoder, video decoder and video output basic code.

Add README on external dependency on pkg-config files for the firmware.

Add --rpi feature to the build system.
rpi: add list of supported video decoder codecs
rpi: fix building
rpi: add some audio decoders
rpi: remove some leftovers from the README
rpi: add omx common helpers
rpi: add license headers