Fix fallback to 2d transforms or standard positioning of offcanvas on legacy browsers, Opera 12 and IE9

IE9 actually supports 2d transforms with -ms prefix, so it will actually use the same code as Opera 12.16

Also a general transform2d mixin along the lines of transform3d makes sense to me.

This code also removes the necessity of using conditional comments in the markup to add .lt-ie10 class to <html> tag.