Change way alpha is used when recoloring symbolics

Change way alpha is used when recoloring symbolics

If the foreground color has an alpha != 1 we used to just pass that into
the svg. This is useful to e.g. render an insensitive icon. However,
that is not an ideal model for symbolics. For instance, if the icon uses
overlapping areas when drawing, expecting these to be opaque then the
transparent color will result in a different alpha value for the overlapping
area. Also, non-foreground symbolic colors are still rendered opaque, and
the recoloring of pngs can't handle transparent colors.

So, instead we extract any alpha from the foreground, render using the
opaque colors and then apply the foreground alpha to the entire result.
This means we get an even transparency, even for other colors, and we
can apply alpha for the pngs too.

https://bugzilla.gnome.org/show_bug.cgi?id=734668