The following CSS would infloop:
@define-color self @self
as it would infinitely lookup the color named "self" and try to resolve
it. This patch adds detection of such cycles to the resolve function by
keeping a list of currently resolving colors in the cycle_list variable.
If a named color references a nonexistant named color, we didn't catch
that error and ended up crashing on a NULL-dereference.
This crashed Boxes, because its CSS referenced values from the theme
that didn't exist in any theme.
Make _gtk_style_provider_private_get_color() return a GtkCssValue (a
GtkCssColorValue to be exact) instead of GtkSymbolicColor.
With this, the symbolic color usage inside GTK is minimized.
