fix issue with selectbox on canvas - Camera

Camera.current doesn't return the correct camera for the
RectTransformUtility.RectangleContainsScreenPoint method. I cached the
parent Canvas component to have access to it's worldCamera property and
use this with the RectangleContainsScreenPoint method.
code formatting
Merge pull request #1 from DoomSamurai/master

Fix issue with selectbox on canvas - Camera