[Qt] Previous web page appears outside content rect

[Qt] Previous web page appears outside content rect
https://bugs.webkit.org/show_bug.cgi?id=78816

Source/WebCore:

Adjust the scissor clipping by -1, as the glScissor() function requires coordinates and
not sizes.

Reviewed by Kenneth Rohde Christiansen.

No new functionality.

* platform/graphics/opengl/TextureMapperGL.cpp:
(WebCore::scissorClip):

Source/WebKit2:

Apply a clip on painted tiles if some of the tiles fall outside of the target rect.

Reviewed by Kenneth Rohde Christiansen.

* UIProcess/qt/LayerBackingStore.cpp:
(WebKit::LayerBackingStore::paintToTextureMapper):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@108407 268f45cc-cd09-0410-ab3c-d52691b4dbfc