Image Widget does not render with custom background

reproduce:
add image widget to bar:

config.py:
```
...
widget.Image(filename="~/icon.png", background="#FF0000"),
...
```
image widget will render with default bar background.

Cause and Fix
The image widget ignores any custom background color given when drawing. see line 77 of libqtile/widget/image.py:
```
self.drawer.clear(self.bar.background)
```
This line should be:
```
self.drawer.clear(self.background or self.bar.background)
```
This will only use the default background if the custom background remains undefined.

Package Version: 0.15.1-1

Note this change is present in the qtile git repository:
https://github.com/qtile/qtile/blob/master/libqtile/widget/image.py