wibox.layout.base.rect_to_device_geometry: Fix for "weird" rotations

The old code transformed the top-left and bottom-right corner of the rectangle
to device space and calculated a rectangle based on these two points. However,
if you rotate a rectangle by 45°, these two points will be directly above each
other and thus the old code would calculate a width of 0.

Fix this by transforming all four corners of the rectangle into device space and
calculating a rectangle based on this.

Signed-off-by: Uli Schlachter <psychon@znc.in>