[TexMap] Don't use GraphicsContext3D in ClipStack

[TexMap] Don't use GraphicsContext3D in ClipStack
https://bugs.webkit.org/show_bug.cgi?id=174776

Reviewed by Carlos Garcia Campos.

Any GraphicsContext3D object that's passed to ClipStack methods is of the
render-to-current-context nature, meaning there's no internally owned GL
context that has to be properly handled and all calls are simply passed to
OpenGL APIs. We should drop such (non-)usage of GraphicsContext3D in favor
of direct OpenGL API invocations.

This patch covers TextureMapper's ClipStack. Call sites to the apply() and
applyIfNeeded() are modified to not pass a reference to any
GraphicsContext3D object. Internally, OpenGL API entrypoints and constants
are used instead of GraphicsContext3D invocations.

No new tests -- no change in behavior.

* platform/graphics/texmap/BitmapTextureGL.cpp:
(WebCore::BitmapTextureGL::clearIfNeeded):
(WebCore::BitmapTextureGL::bindAsSurface):
* platform/graphics/texmap/ClipStack.cpp:
(WebCore::ClipStack::apply):
(WebCore::ClipStack::applyIfNeeded):
* platform/graphics/texmap/ClipStack.h:
* platform/graphics/texmap/TextureMapperGL.cpp:
(WebCore::TextureMapperGL::bindDefaultSurface):
(WebCore::TextureMapperGL::beginScissorClip):
(WebCore::TextureMapperGL::beginClip):
(WebCore::TextureMapperGL::endClip):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@220399 268f45cc-cd09-0410-ab3c-d52691b4dbfc