The initial target color state is the color state of the stage view
being painted to. If we're painting to an arbitrary framebuffer, it's
currently hard coded to sRGB/electrical.

The content color state is not set on construction, but when starting to
paint, it's set to the color state of the stage itself. Whenever an
actor is painted, it'll set the color state to the color state of
itself. The intention is that offscreen rendering pushes a target color
state that causes painting to it to not necessarily be in the stage view
color state.

Pass color state with offscreen framebuffer, as this avoids hard coding
sRGB in the lower level bits of paint contexts. It's still practically
hard coded, only that it's derived from somewhere else (e.g. the stage
or window actor).

Nothing is actually using this yet, but will eventually.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3433>