Switching between smooth and flat shading in OBJ preview

[sczerwinski]

glShadeModel should be changed based on the value of s flag:

• s off – GL_FLAT
• s 1 – GL_SMOOTH

This change might occur multiple times while drawing a single model.

[sczerwinski]

Should this change mean switching to another shader (performance improvement) or should it be reflected in the mesh loaded from an OBJ file (implementation simplicity)?

[sczerwinski]

Since there are multiple smoothing groups possible (see #149), expected behaviour of the 3D preview is not obvious.

Also, according to OBJ file type documentation:

However, vertex normals override smoothing information for Image.

Seems like smoothing groups are only useful for automated normal calculations, covered in #36.