QOpenGLFramebufferObject needs an API for taking the texture (with ownership)

Add QOpenGLFramebufferObject::takeTexture() to complement the existing texture(). In some cases it would be very useful if the texture could be detached from the QOpenGLFramebufferObject instance, meaning that the ownership of the texture were transfered to the called and the next bind() would then automatically create and associate a new texture.