Shader generator errors out with some material property combinations

[Juha Jeronen]

In the function ShaderGenerator::synthesize_shader(), contained in panda/src/pgraphnodes/shaderGenerator.cxx, there are some code paths that cause tot_diffuse to be used without being defined, specifically when light ramping is enabled. One of these is when a material has specular, but no diffuse or ambient.

This causes the generated shader to fail, reverting to the default renderer for the affected material, and printing an error message in the terminal.

The findings and a fix are reported here:

http://www.panda3d.org/forums/viewtopic.php?f=4&t=16329

Attached is a patch (against 1.8.1) containing a fix along the lines suggested by rdb in the forum thread.

[rdb (rdb)]

Checked in a fix, thanks.