Default fill_opacity to 1 when fill_color is set

[k4pran]

Enhancement proposal

Currently fill_opacity for VMobject defaults to 0 which makes sense, but it doesn't make sense for it to remain 0 when the user sets the fill, as fill then doesn't really have any effect:

class Example(Scene):
    def construct(self):
        rect = Rectangle(fill_color=GREEN)
        self.add(rect)

I propose that when fill is set opacity should then default to 1 as in the absence of fill_opacity passed by the user it is most likely what they would expect to happen.

Additional comments

[naveen521kk]

lgtm