https://bugs.webkit.org/show_bug.cgi?id=123106

Reviewed by Simon Fraser.

* WebProcess/WebPage/mac/PlatformCALayerRemote.cpp:
(PlatformCALayerRemote::ensureBackingStore):
(PlatformCALayerRemote::setNeedsDisplay):
(PlatformCALayerRemote::setContentsScale):
Inform the backing store of the contentsScale, and when it changes.

* WebProcess/WebPage/mac/RemoteLayerBackingStore.h:
(WebKit::RemoteLayerBackingStore::scale): Added.
(WebKit::RemoteLayerBackingStore::mapToContentCoordinates): Added.
* WebProcess/WebPage/mac/RemoteLayerBackingStore.mm:
(RemoteLayerBackingStore::RemoteLayerBackingStore):
(RemoteLayerBackingStore::encode):
(RemoteLayerBackingStore::decode):
(RemoteLayerBackingStore::mapToContentCoordinates):
Factor out flipping so we can use it for repaints in multiple places.

(RemoteLayerBackingStore::setNeedsDisplay):
(RemoteLayerBackingStore::display):
Create a buffer scaled up by contentsScale, take the scale into account
when copying clean bits from the front buffer, and apply the scale before
painting into the back buffer.

* WebCore.exp.in:
Export FloatRect::scale.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@157728 268f45cc-cd09-0410-ab3c-d52691b4dbfc