glgsg: Fix 1 texture stage limit when setting `gl-version 3 2` or higher

Fixes #1404

panda/src/glstuff/glGraphicsStateGuardian_src.cxx

@@ -11623,7 +11623,17 @@ set_state_and_transform(const RenderState *target,
   if (_target_rs->get_attrib(texture_slot) != _state_rs->get_attrib(texture_slot) ||
       !_state_mask.get_bit(texture_slot)) {
     PStatGPUTimer timer(this, _draw_set_state_texture_pcollector);
-    determine_target_texture();
+    if (has_fixed_function_pipeline() ||
+        _current_shader == nullptr ||
+        _current_shader == _default_shader) {
+      determine_target_texture();
+    } else {
+      // If we have a custom shader, don't filter down the list of textures.
+      _target_texture = (const TextureAttrib *)
+        _target_rs->get_attrib_def(TextureAttrib::get_class_slot());
+      _target_tex_gen = (const TexGenAttrib *)
+        _target_rs->get_attrib_def(TexGenAttrib::get_class_slot());
+    }
     do_issue_texture();
 
     // Since the TexGen and TexMatrix states depend partly on the particular