Allow SWS_CS_BT2020 to Colorspace

Overview

FFmpeg support BT2020 colorspace，the enum is SWS_CS_BT2020。

Existing FFmpeg API

in file libswscale/swscale.h

#define SWS_CS_ITU709         1
#define SWS_CS_FCC            4
#define SWS_CS_ITU601         5
#define SWS_CS_ITU624         5
#define SWS_CS_SMPTE170M      5
#define SWS_CS_SMPTE240M      7
#define SWS_CS_DEFAULT        5
#define SWS_CS_BT2020         9

Expected PyAV API

in file av/video/reformatter.pyx

Colorspace = define_enum('Colorspace', __name__, (

    ('ITU709', lib.SWS_CS_ITU709),
    ('FCC', lib.SWS_CS_FCC),
    ('ITU601', lib.SWS_CS_ITU601),
    ('ITU624', lib.SWS_CS_ITU624),
    ('SMPTE170M', lib.SWS_CS_SMPTE170M),
    ('SMPTE240M', lib.SWS_CS_SMPTE240M),
    ('DEFAULT', lib.SWS_CS_DEFAULT),
    ('BT2020', lib.SWS_CS_BT2020),

    # Lowercase for b/c.
    ('itu709', lib.SWS_CS_ITU709),
    ('fcc', lib.SWS_CS_FCC),
    ('itu601', lib.SWS_CS_ITU601),
    ('itu624', lib.SWS_CS_SMPTE170M),
    ('smpte240', lib.SWS_CS_SMPTE240M),
    ('default', lib.SWS_CS_DEFAULT),
    ('bt2020', lib.SWS_CS_BT2020),

))

in file include/libswscale/swscale.pxd

cdef int SWS_CS_BT2020

Example:

frame.reformat(src_colorspace='bt2020', dst_colorspace=’itu709', format="bgr24")