workaround for NVidia 361.43 driver bug

This driver returns internal/synthetic uniform shaders when
asked for the list of active uniforms. They all start with
"_main_" and have names like "_main_0_gp5fp[0]". We need to
ignore these, otherwise, Panda will later crash with failed
assertions when trying to set this unknown uniform.

panda/src/glstuff/glShaderContext_src.cxx

@@ -687,6 +687,15 @@ reflect_uniform(int i, char *name_buffer, GLsizei name_buflen) {
   _glgsg->_glGetActiveUniform(_glsl_program, i, name_buflen, NULL, &param_size, &param_type, name_buffer);
   GLint p = _glgsg->_glGetUniformLocation(_glsl_program, name_buffer);
 
+
+  // Some NVidia drivers (361.43 for example) (incorrectly) include "internal" uniforms in 
+  // the list starting with "_main_" (for example, "_main_0_gp5fp[0]")
+  // we need to skip those, because we don't know anything about them
+  if (strncmp(name_buffer, "_main_", 6) == 0) {
+    GLCAT.warning() << "Ignoring uniform " << name_buffer << " which may be generated by buggy Nvidia driver.\n";
+    return;
+  }
+
   if (GLCAT.is_debug()) {
     GLCAT.debug()
       << "Active uniform " << name_buffer << " with size " << param_size