pgssubdec: fix subpicture output colorspace and range

Functionality used before didn't widen the values from limited to
full range. Additionally, now the decoder uses BT.709 where it
should be used according to the video resolution.

Default for not yet set colorimetry is BT.709 due to most observed
HDMV content being HD.

BT.709 coefficients were gathered from the first two parts of BT.709
to BT.2020 conversion guide in ARIB STD-B62 (Pt. 1, Chapter 6.2.2).
They were additionally confirmed by manually calculating values.

Fixes #4637
