color: add from sRGB to blending color transformation

This is needed when the compositor produces any content internally:
- the lines in triangle fan debug
- the censoring color fill (unmet HDCP requirements)

Solid color surfaces do not need this special-casing because
weston_surface is supposed to carry color space information, which will
get used in gl_shader_config_init_for_view().

This makes sure the internally produced graphics fit in, e.g on a
monitor in HDR mode.

For now, just ensure there is an identity transformation. Actual
implementations in GL-renderer will follow later.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>