Make faces shading correct for all possible modes.

Skip shading for lightsources, they shouldnt be shaded at all.
This also fixes issue liquids sources and flowing top faces have different brightness.
And some speedup: 
- top faces of nodes are not shaded at all
- when add shading we dont have to multiply blue channel - it holds lightsource values and is 0 for regular nodes.

Note: there are still issues with animated liquids but not caused by this code, i mean shading.
Theres a bug in maplock_mesh::animate that lets calling finalColorBlend when shaders are on, also
flowing liquids drawtype has some code to change light for liquids that are lightsources, while liquid sources drawtype doesnt have. But thats subject for another pull.