CompoundButtons such as Checkboxes, when are allowed to shrink (flexShrink = 1f), during the recalculation can end up being shrunk more than what I would consider their "minimum size" since the negative space is calculated for all FlexItems in the shrinkFlexItems() method.

In this PR I try to prevent these components to get completely shrunk by considering the minimum size of their button drawable. In this way, the drawable size will determine the minimum size of the widget unless a minWidth or minHeight are specified by the user.