glgsg: fix shader inputs not being updated in esoteric case

Specifically, this happens if you have a custom ShaderGenerator that assigns shader inputs via the ShaderAttrib.  If two objects then both have the auto-shader turned on, and the shader generator generates ShaderAttribs with the same shaders but different inputs, the inputs will not be updated properly for one of the objects.

panda/src/glstuff/glShaderContext_src.cxx

@@ -2042,10 +2042,6 @@ set_state_and_transform(const RenderState *target_rs,
         target_rs->get_attrib(MaterialAttrib::get_class_slot())) {
       altered |= Shader::SSD_material;
     }
-    if (state_rs->get_attrib(ShaderAttrib::get_class_slot()) !=
-        target_rs->get_attrib(ShaderAttrib::get_class_slot())) {
-      altered |= Shader::SSD_shaderinputs;
-    }
     if (state_rs->get_attrib(FogAttrib::get_class_slot()) !=
         target_rs->get_attrib(FogAttrib::get_class_slot())) {
       altered |= Shader::SSD_fog;
@@ -2069,6 +2065,11 @@ set_state_and_transform(const RenderState *target_rs,
     _state_rs = target_rs;
   }
 
+  if (_shader_attrib.get_orig() != _glgsg->_target_shader || _shader_attrib.was_deleted()) {
+    altered |= Shader::SSD_shaderinputs;
+    _shader_attrib = _glgsg->_target_shader;
+  }
+
   // Is this the first time this shader is used this frame?
   int frame_number = ClockObject::get_global_clock()->get_frame_count();
   if (frame_number != _frame_number) {

panda/src/glstuff/glShaderContext_src.h

@@ -76,6 +76,7 @@ private:
   CPT(TransformState) _camera_transform;
   CPT(TransformState) _projection_transform;
   CPT(ColorAttrib) _color_attrib;
+  WCPT(ShaderAttrib) _shader_attrib;
 
 /*
  * struct ParamContext { CPT(InternalName) _name; GLint _location; GLsizei