[Mapcss] z-index

[Paul Hartmann]

Hi,

There is a new way to deal with z-index in JOSM:

I've added an additional property "major-z-index", which takes default
values depending on the style element:

area - 1
line-casing - 2
line - 3
point - 4
text - 5

the major-z-index has higher priority than the z-index, so the z-index
is only considered, when the major-z-index of 2 objects is identical.

The advantage of this system is, that painting order is not
hard-coded, but the major-z-index can be overridden by the style
author just like any other property. The vast majority of users can
ignore this property and still get good results.

Paul

PS: I've also adapted the semantics of casing-width, to match that of
most other MapCSS implementations.