Before this patch, we were painting into our normal backing store
HBITMAP, wrapping it in a CGImage, and handing it off to Core
Animation. This had at least two disadvantages:
   1) The bitmap could be modified while Core Animation was using it.
   2) It introduced extra complexity.

When entering accelerated compositing mode, WebView now creates a
GraphicsLayer to draw into. This GraphicsLayer sits between the root
layer (owned by WKCACFLayerRenderer) and the RenderView's layer. When
WebView invalidates, it just calls setNeedsDisplayInRect on its
GraphicsLayer. When WebView paints, it just tells its
WKCACFLayerRenderer to paint, which will call back to WebView to draw
into the GraphicsLayer if it has a dirty region. This is very similar
to the current implementation of LayerBackedDrawingArea in WebKit2.

Fixes <http://webkit.org/b/52582> WebView should paint directly into a
GraphicsLayer when in accelerated compositing mode

Reviewed by Simon Fraser and Chris Marrin.

WebCore:

Remove special-cased support for providing content for the root layer

Clients will just have to provide content through the normal
GraphicsLayer channels now!

* platform/graphics/win/WKCACFLayerRenderer.cpp:
(WebCore::WKCACFLayerRenderer::WKCACFLayerRenderer): Removed
initialization of m_backingStoreDirty, which has itself been removed.
(WebCore::WKCACFLayerRenderer::setNeedsDisplay): Moved code to schedule
a sync from here to syncCompositingStateSoon. We only need to call
renderSoon if we don't call syncCompositingStateSoon; the latter
function calls the former.
(WebCore::WKCACFLayerRenderer::paint): Removed code to handle
m_backingStoreDirty. We don't want to know anything about clients'
backing stores.
(WebCore::WKCACFLayerRenderer::syncCompositingStateSoon): Added. Code
came from setNeedsDisplay.

* platform/graphics/win/WKCACFLayerRenderer.h:
Removed setRootContents[AndDisplay], setBackingStoreDirty, and
m_backingStoreDirty. Made paint() public so that clients can force a
synchronous render (e.g., when handling WM_PAINT).

WebKit/win:

* WebCoreSupport/WebChromeClient.cpp:
(WebChromeClient::attachRootGraphicsLayer):
(WebChromeClient::scheduleCompositingLayerSync):
Updated for WebView changes.

* WebView.cpp:
(WebView::repaint): When using accelerated compositing, just invalidate
our backing layer. This matches the behavior of LayerBackedDrawingArea.
(WebView::deleteBackingStore): Removed accelerated compositing code.
The WKCACFLayerRenderer doesn't need to know about our backing store
anymore; we don't use it at all when in accelerated compositing mode.
(WebView::addToDirtyRegion): When using accelerated compositing, just
invalidate our backing layer.
(WebView::scrollBackingStore): Added an assertion that this isn't
called in accelerated compositing mode.
(WebView::sizeChanged): Update our backing layer's size, too, and
invalidate it.
(WebView::updateBackingStore): Added an assertion that this isn't
called in accelerated compositing mode.
(WebView::paint): If we're in accelerated compositing mode, sync our
compositing state. If we're *still* in accelerated compositing mode,
just tell our WKCACFLayerRenderer to paint and clear our dirty region.
(The later changes in this function are just un-indenting code that
used to be inside an if.)

(WebView::paintIntoBackingStore):
(WebView::paintIntoWindow):
Added assertions that these aren't called in accelerated compositing
mode.

(WebView::WebViewWndProc): Updated WM_XP_THEMECHANGED handling for
removal of setRootLayerNeedsDisplay.
(WebView::setRootChildLayer): Changed to take a GraphicsLayer. We now
set the layer as a child of our own backing layer.
(WebView::scheduleCompositingLayerSync): Just call through to
WKCACFLayerRenderer.
(WebView::setAcceleratedCompositing): Create our backing layer and set
it as the child of WKCACFLayerRenderer's root layer.
(WebView::notifyAnimationStarted): Added. We never expect this
GraphicsLayerClient override to be called, as we don't use animations
on our backing layer.
(WebView::notifySyncRequired): Added. Just schedule a sync.
(WebView::paintContents): Added. Just clip and paint!

(WebView::showDebugBorders):
(WebView::showRepaintCounter):
Added. These just call through to Settings.

(WebView::syncCompositingState): Changed to first update layout, then
sync state for our backing layer, then sync WebCore's state. This
matches LayerBackedDrawingArea.

* WebView.h: WebView now implements the GraphicsLayerClient interface.
Removed setRootLayerNeedsDisplay; it's been replaced by calling
setNeedsDisplay on our backing layer and calling
syncCompositingStateSoon on WKCACFLayerRenderer as needed. Removed
updateRootLayerContents; that function was used to pass our backing
store to Core Animation, which we no longer do. Added m_backingLayer.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@75987 268f45cc-cd09-0410-ab3c-d52691b4dbfc