For various reasons, we want to be pushing the layer's local clip rect
outside of the stacking context rather than inside it. Not only is this
more correct with respect to the semantics of the layer tree, we also
need it in order to properly handle fixed-positioning of layers with
async scrolling.
This patch does the bulk of the work to make this happen. Most of the code
in the individual layer classes to process the layer's local clip rect
is removed, and instead a function in ScrollingLayersHelper is added to
deal with it. There are a couple of places that individual layer classes
still handle this but those will be removed in future patches. Note that
the individual layer classes still need to provide a clip rect of some
sort in order to push their display items, and now they simply use their
visible region bounds for this purpose.
MozReview-Commit-ID: IBmfUdJwYx1
This adds an RAII helper and uses it in RenderLayer functions. When APZ
is enabled, the RAII helper pushes a scrolling clip for each scrollable
metrics on the layer. It also pops off the scrolling clips on
destruction. Note that this should happen before any other things are
pushed into the WR display list for the rendering of a layer, since
those things should be subjected to the enclosing scrolling clips.
If APZ is disabled, this skips pushing the scrolling clips.
MozReview-Commit-ID: 1qv9egKbbok
