[Texmap] Allow TextureMapperGL to work without a GraphicsContext

[Texmap] Allow TextureMapperGL to work without a GraphicsContext
https://bugs.webkit.org/show_bug.cgi?id=68980

Reviewed by Andreas Kling.

Perform a null-check for m_context and initialize that variable.
If a graphics-context doesn't exist, we don't need to reset it.

No new functionality so no new tests.

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

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@96211 268f45cc-cd09-0410-ab3c-d52691b4dbfc