Remove MozMousePixelScroll handler because we no longer need it.

Horizontal and vertical scrolling now work on the same scrollbox, so the built-in behavior works just fine!
Render the histogram in HiDPI when possible.
Merge pull request #29 from mstange/tree-scrolling

Remove MozMousePixelScroll handler because we no longer need it.
Merge pull request #30 from mstange/hidpi-histogram

Render the histogram in HiDPI when possible.