Reviewed by Sam Weinig.

        Begin stubbing out some more BackingStore member functions
        https://bugs.webkit.org/show_bug.cgi?id=52323

        * UIProcess/BackingStore.cpp:
        (WebKit::BackingStore::~BackingStore):
        This was accidentally made into a constructor; it should be a destructor.

        * UIProcess/DrawingAreaProxyImpl.cpp:
        (WebKit::DrawingAreaProxyImpl::paint):
        Call the backing store.

        (WebKit::DrawingAreaProxyImpl::update):
        Incorporate the update.

        (WebKit::DrawingAreaProxyImpl::incorporateUpdate):
        Create the backing store if needed and incorporate the update.

        * UIProcess/mac/BackingStoreMac.mm: Added.
        (WebKit::BackingStore::platformInitialize):
        Create a bitmap context.

        (WebKit::BackingStore::paint):
        (WebKit::BackingStore::incorporateUpdate):
        Add stubs.

        * WebKit2.xcodeproj/project.pbxproj:
        Add BackingStoreMac.mm


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@75640 268f45cc-cd09-0410-ab3c-d52691b4dbfc