More options for customizing the blurring effect

* Added a monochrome mode.
* Made overriding the alpha value optional.
* Added a min and max brightness parameter, to limit the min/max brightness of the resulting blurred image -> helps when you want to present text over the blurred image and need to make sure that the colors won't collide and make the text unreadable.

Signed-off-by: Or Sharir <or@sharir.org>