Also, fix the fitting logic so that the result width and height are fitted in the available space. Previously the result dimensions can be larger than the input, but cropped in e7a21947e6/lib/wibox/widget/base.lua (L547). But still it can cause problems e.g. when used inside wibox.container.place.
When less space is available than was asked, systray:draw() has to
compute the right base size so that all the icons fit into the available
space. This computation so far ignored the icon spacing, resulting in a
too large base size.
Fixes: https://github.com/awesomeWM/awesome/issues/2981
Signed-off-by: Uli Schlachter <psychon@znc.in>