Add useBackdropFilter

[miguelpruivo]

Using BackdropFilter has a big impact in performance specially when using multiple CoachMarks on lower end devices. It has a nice effect, however, in some cases using just an alpha color on background is enough and may result in a better experience.

Hence, this PR aims to add the useBackdropFilter property which defaults to true but lets developers remove the BackdropFilter if they want to.

Thank you!