Parallax calculation should use the center of the viewport

[thelucre]

Problem:
Translations are dramatically different for elements depending on where they are in the entire page layout. Using absY or not doesn't help.

Expected:
Parallax translations should be relative to the element's position to the window viewport, not the entire page height. An element at dead center of the window viewport should effectively have 0 translation applied, so layouts can be controlled.

[thelucre]

The Rellax library has a similar feature, see:
dixonandmoe/rellax#67 (comment)