Default fill_opacity to 1 when fill_color is set

Nobody has claimed this yet.

## 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:

```py
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

Start by tracing how VMobject handles fill_color and fill_opacity during construction, using the Rectangle example in the issue as the expected entry point. The change is done when setting fill_color without an explicit fill_opacity produces a visible fill with opacity 1, while an explicitly supplied opacity remains respected.

Written by the indexing model from the issue text.