Add Safari overlay position fix (#178)

Fix for blurry image rendering in OS X Safari

src/css/_fluidbox.scss

@@ -21,9 +21,10 @@ $fluidbox__animation-bg-color: $fluidbox__overlay-bg-color !default;
 	opacity: 0;
 	pointer-events: none;
 	position: fixed;
-	top: 0;
+	/* Negative top and bottom is to fix some Safari cases where image got blurry */
+	top: -100%;
 	left: 0;
-	bottom: 0;
+	bottom: -100%;
 	right: 0;
 	/* Transition time for overlay is halved to ensure that flickering doesn't happen */
 	transition: all $fluidbox__transition-duration ease-in-out;
@@ -101,4 +102,4 @@ $fluidbox__animation-bg-color: $fluidbox__overlay-bg-color !default;
 	&::before {
 		animation: fluidboxLoading 1s 0s infinite ease-in-out forwards;
 	}
-}
+}