JoystickThumbPad: Fix invalid axis value calculation
JoystickThumbPad: Fix invalid axis value calculation

When the item is not visible, the width and height are not valid,
resulting in wrong output values

Signed-off-by: Patrick José Pereira <c2b7559029b4032a4ce95718d7d3bc36c8a1d7c1@gmail.com>

Hunter522/qgroundcontrol,Hunter522/qgroundcontrol,Hunter522/qgroundcontrol,Hunter522/qgroundcontrol,Hunter522/qgroundcontrol,Hunter522/qgroundcontrol