More specific description of screenWidth parameter
Correctly round screen width

When zooming my 3200px display width by 1.5x, the window width is rounded to
2133. Multiplying this back with 1.5x yielded 3199.5, causing AutoHiDPI to
(incorrectly) consider my window non-HiDPI and zooming it back to 1x.

This patch correctly rounds the window size back.
Merge pull request #3 from raboof/master

Fix rounding error (and docs)