[TextureMapper] Clean up BitmapTextureGL construction

[TextureMapper] Clean up BitmapTextureGL construction
https://bugs.webkit.org/show_bug.cgi?id=168909

Reviewed by Carlos Garcia Campos.

Have the BitmapTextureGL constructor accept an rvalue reference pointer
to the GraphicsContext3D object. A static create() method is also added
to help with constructing these objects. Construction sites are updated
appropriately.

The BitmapTextureGL constructor is further cleaned up by moving default
member initializations together with the member declarations.

* platform/graphics/gstreamer/MediaPlayerPrivateGStreamerBase.cpp:
(WebCore::MediaPlayerPrivateGStreamerBase::pushTextureToCompositor):
* platform/graphics/texmap/BitmapTextureGL.cpp:
(WebCore::BitmapTextureGL::BitmapTextureGL):
* platform/graphics/texmap/BitmapTextureGL.h:
* platform/graphics/texmap/BitmapTexturePool.cpp:
(WebCore::BitmapTexturePool::createTexture):
* platform/graphics/texmap/TextureMapperGL.cpp:
(WebCore::TextureMapperGL::createTexture):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@213044 268f45cc-cd09-0410-ab3c-d52691b4dbfc