ui/layout: restore Flex.Rigid behaviour

Broken by 5f2adf9b2f.

Clamp to 0 while we're here.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/layout/flex.go

@@ -81,7 +81,10 @@ func (f *Flex) begin(mode flexMode) {
 func (f *Flex) Rigid() Constraints {
 	f.begin(modeRigid)
 	mainc := axisMainConstraint(f.Axis, f.cs)
-	mainMax := mainc.Max
+	mainMax := mainc.Max - f.size
+	if mainMax < 0 {
+		mainMax = 0
+	}
 	return axisConstraints(f.Axis, Constraint{Max: mainMax}, axisCrossConstraint(f.Axis, f.cs))
 }