Vertical background offset is calculated incorrectly

[Gargaj]

reveal.js/js/controllers/backgrounds.js

Line 417 in ea3d7aa

verticalSlides = this.Reveal.getVerticalSlides();

getVerticalSlides() returns the total number of vertical slides, even if they are parallel to each other.

This means that a slide layout like this

A-C-F-H-L
| | | |
B D G I
  |   |
  E   J
      |
      K

will return 12, rather than 4, which means a background with the height of 1200px will only scroll down 100px, rather than 300px as intended.