Insert #line statements in shader program.

This makes the GLSL compiler issue correct line numbers on errors
even in the presence of the other preprocessor lines inserted by
QOpenGLShaderProgram/QGLShaderProgram.

Change-Id: Ief4fc1dd1e94bb2b3a1ad13fbaf62186910a4994
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>