glgsg: Disable use of GL_ARB_shading_language_420pack

spirv-cross uses this to generate the bindings, but we rebind everything through code anyway, so it's not really necessary, and it causes problems on NVIDIA drivers with GLSL 1.30 output due to a driver bug

panda/src/glstuff/glShaderContext_src.cxx

@@ -3568,6 +3568,7 @@ attach_shader(const ShaderModule *module, Shader::ModuleSpecConstants &consts) {
       options.es = false;
 #endif
       options.vertex.support_nonzero_base_instance = false;
+      options.enable_420pack_extension = false;
       compiler.set_common_options(options);
 
       // At this time, SPIRV-Cross doesn't add this extension automatically.