Allow creation of QOpenGLShaderProgram without a current context

Allow creation of QOpenGLShaderProgram without a current context

This change prevents the qWarning() in QOpenGLFunctions from being
triggered when creating a QOpenGLShaderProgram object without a current
QOpenGLContext. This is a common case especially when the shader program
is a member variable of a class.

It now relies upon the QOpenGLShaderProgram::init() function to resolve
the GL function pointers when the shader program is first used which of
course should be done with an active context.

Change-Id: Ib7dc345593f2a70d72ef8f650151cc90001058ca
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>