These can live in the <stop> element attributes, or in its style="..."
attribute.

Neither of them inherits by default; they have to be explicitly
specified as "inherit" to do that.

Inheritance is from the gradient's parent element, not from where it is
referenced, hence the call to rsvg_state_reconstruct().

Also, stop-color can be currentColor.

We now have explicit enums to indicate all of these cases.  Previously,
all the meanings were overloaded into state->has_stop_color and
state->has_stop_opacity, and those didn't handle all cases correctly.

Added the hexchat.svg icon to the tests, which is how I caught the lack
of proper handling for style="stop-color: ...;".