FLT_MIN is the smallest *positive* number above 0 that can be
represented as floating point number. If this is used to initialize the
maximum x/y coordinates of a rectangle, this will always be used if all
x/y coordinates of the rectangle are negative. This means that picking
at 0,0 will always be a hit for such rectangles.

Since mutter creates such a window for server side decorations on X11,
this window will always be picked at 0,0 preventing clicking/hovering
the activities button in gnome-shell at that coordinate.

Fixes GNOME/mutter#893