<rdar://problem/10891801> BackingStore::scroll() unnecessarily copies pixels around

<rdar://problem/10891801> BackingStore::scroll() unnecessarily copies pixels around
https://bugs.webkit.org/show_bug.cgi?id=78976

Reviewed by Anders Carlsson.

Rather than move pixels in the backing store in response to scrolling, we can maintain a
mapping, for the most recently scrolled rect, from backing store coordinates to view
client coordinates.

* UIProcess/BackingStore.h:
* UIProcess/mac/BackingStoreMac.mm:
(WebKit::BackingStore::performWithScrolledRectTransform): Added. Given a block to be
performed on a rect, divides the rect into parts such that for each part the mapping from
backing store coordinates to client coordinates is a (uniform) translation, and performs
the block on that part, passing it the translation that applies to the part.
(WebKit::BackingStore::resetScrolledRect): Added. Copies everything in the scrolled rect
back to where it should be under the identity map, and resets the scrolled rect and offset.
(WebKit::BackingStore::paint): Changed to call through performWithScrolledRectTransform().
(WebKit::BackingStore::incorporateUpdate): Ditto.
(WebKit::BackingStore::scroll): Now instead of copying pixels, just updates the scrolled
rect and offset.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@108181 268f45cc-cd09-0410-ab3c-d52691b4dbfc