[Texmap] Use glScissors for clipping in TextureMapperGL when possible

[Texmap] Use glScissors for clipping in TextureMapperGL when possible
https://bugs.webkit.org/show_bug.cgi?id=77575

Reviewed by Martin Robinson.

Source/WebCore:

Maintain a clipping stack, that helps us use stencils in conjunction with scissors.
We apply scissors when the clip region is rectalinear, and stencil when it's not.

No behavior changes so no new tests.

* platform/graphics/opengl/TextureMapperGL.cpp:
(SharedGLData):
(WebCore::TextureMapperGLData::SharedGLData::SharedGLData):
(WebCore::TextureMapperGL::drawTexture):
(WebCore::TextureMapperGL::bindSurface):
(WebCore):
(WebCore::scissorClip):
(WebCore::TextureMapperGL::beginScissorClip):
(WebCore::TextureMapperGL::endScissorClip):
(WebCore::TextureMapperGL::beginClip):
(WebCore::TextureMapperGL::endClip):
* platform/graphics/opengl/TextureMapperGL.h:
(TextureMapperGL):

Source/WebKit2:

Instead of applying the scissor clip in QQuickWebPage, we trickle it down to
TextureMapperGL, and apply it there as part of beginClip(). All direct GL operations are
now cleaned out of QQuickWebPage.

* UIProcess/API/qt/qquickwebpage.cpp:
(QQuickWebPagePrivate::paintToCurrentGLContext):
* UIProcess/DrawingAreaProxy.h:
(WebKit::DrawingAreaProxy::paintToCurrentGLContext):
* UIProcess/DrawingAreaProxyImpl.cpp:
(WebKit::DrawingAreaProxyImpl::paintToCurrentGLContext):
* UIProcess/DrawingAreaProxyImpl.h:
(DrawingAreaProxyImpl):
* UIProcess/LayerTreeHostProxy.h:
(LayerTreeHostProxy):
* UIProcess/qt/LayerTreeHostProxyQt.cpp:
(WebKit::LayerTreeHostProxy::paintToCurrentGLContext):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@106524 268f45cc-cd09-0410-ab3c-d52691b4dbfc