[Cogl] CoglGst fixes and features

[Lionel Landwerlin]

Hi all,

I'm reposting a couple of fixes (1 & 2) that haven't made it to 1.18 yet.

Patch 3 is a rework of the YUV <-> sRGB color conversion. It turns out
that depending on the colorimetry format used when encoding to a YUV
format, you need a different conversion matrix when converting to
sRGB. This patch adds support for BT.601 (SD) and BT.709 (HD)
colorimetry formats.

Patch 4 adds the video balance interface support. The purpose of this
is to prevent playbin to add a software element in the pipeline for
doing color balance when using a hardware decoder (even though we
don't support HW decode yet...).

Cheers,

-
Lionel