The text cursor follows slightly different "intuitive" rules than the
regular hit testing. Clicking past the right edge of a text box should
still "hit" the text box, and place the cursor at its end, for example.
We solve this by adding a HitTestType enum that is passed to hit_test()
and determines whether past-the-edge candidates are considered.
StyleProperties is really only the specified "input" to what eventually
becomes the used/computed style we use for layout and painting.
Unlike StyleProperties, LayoutStyle will have strongly typed values for
everything it contains (i.e no CSS::ValueID or strings, etc.)
This first patch moves z-index into LayoutStyle.
The stacking context tree doesn't affect layout at all, so let's move
it into the Painting/ directory. I'm not sure yet if it's worth going
for a fullly separate painting tree. So far I'm thinking a stacking
context tree with pointers into the layout tree might be enough.
"Paint" matches what we call this in the rest of the system. Let's not
confuse things by mixing paint/render/draw all the time. I'm guilty of
this in more places..
Also rename RenderingContext => PaintContext.
