Let QSGPlainTexture support shutting down without a GL context.

Change-Id: Iae934e4d9e91f4ea21dd5bf27c4fafc5d481fb0a
Task-number: QTBUG-41278
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

src/quick/scenegraph/util/qsgtexture.cpp

@@ -563,7 +563,7 @@ QSGPlainTexture::QSGPlainTexture()
 
 QSGPlainTexture::~QSGPlainTexture()
 {
-    if (m_texture_id && m_owns_texture)
+    if (m_texture_id && m_owns_texture && QOpenGLContext::currentContext())
         QOpenGLContext::currentContext()->functions()->glDeleteTextures(1, &m_texture_id);
 }