ass_mp: don't use --subcp for --ass-styles

The --ass-styles option is implemented by calling ass_read_styles().
This function can take a codepage (so libass will use iconv to convert
it). This was implemented before our --subcp option was changed, and
this code was not updated. Now libass fails opening iconv, because
--subcp is not always (and not by default) a valid iconv codepage.

Just always pass NULL, which means the file passed to --ass-styles must
be in UTF-8. The --ass-styles option is a fringe option anyway (and will
destroy your subtitles), so having codepage support for it isn't
important at all.
vo_opengl, cocoa: allow to autoselect a color profile

This commit adds support for automatic selection of color profiles based on
the display where mpv is initialized, and automatically changes the color
profile when display is changed or the profile itself is changed from
System Preferences.

@UliZappe was responsible with the testing and implementation of a lot of this
commit, including the original implementation of `cocoa_get_icc_profile_path`
(See #594).

Fixes #594
vo_corevideo: remove unused variable
gl_lcms: fix build when lcms2 is not available

Was broken in b0ee9334e.
video: Add BT.2020-NCL colorspace and transfer function

Source: http://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.2020-0-201208-I!!PDF-E.pdf
video: Add support for non-BT.709 primaries

This add support for reading primary information from lavc, categorized
into BT.601-525, BT.601-625, BT.709 and BT.2020; and passes it on to the
vo. In vo_opengl, we always generate the 3dlut against the wider BT.2020
and transform our source into this colorspace in the shader.
options: Expose --primaries to the user
video: Support BT.2020 constant luminance system
video: Generate an accurate CMS matrix instead of hard-coding

This also avoids an extra matrix multiplication when using :srgb, making
that path both more efficient and also eliminating more hard-coded
values.

In addition, the previously hard-coded XYZ to RGB matrix will be
dynamically generated.
video: Better support for XYZ input

With this change, XYZ input is directly converted to the output
colorspace wherever possible, and to the colorspace specified by the
tags and/or --primaries option, otherwise.

This commit also restructures some of the CMS code in gl_video.c to
hopefully make it clearer which decision is being done where and why.
video: Refactor rendering intent for DCP XYZ and :srgb

Notably, we now conform to SMPTE 428-1-2006 when decoding XYZ12 input,
and we can support rendering intents other than colorimetric when
converting between BT.709 and BT.2020, like with :srgb or :icc-profile.