Fix T99528: EEVEE: Regression: Faulty shaders when using Volume Info node

Workaround the issue by adding an intermediate function. This is usually
the case when working with attributes.

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D15860

source/blender/gpu/shaders/material/gpu_shader_material_attribute.glsl

@@ -12,6 +12,16 @@ void node_attribute_temperature(vec4 attr, out vec4 out_attr)
   out_attr.w = 1.0;
 }
 
+void node_attribute_density(vec4 attr, out float out_attr)
+{
+  out_attr = attr.x;
+}
+
+void node_attribute_flame(vec4 attr, out float out_attr)
+{
+  out_attr = attr.x;
+}
+
 void node_attribute(
     vec4 attr, out vec4 outcol, out vec3 outvec, out float outf, out float outalpha)
 {

source/blender/nodes/shader/nodes/node_shader_volume_info.cc

@@ -25,9 +25,11 @@ static int node_shader_gpu_volume_info(GPUMaterial *mat,
   }
   if (out[1].hasoutput) {
     out[1].link = GPU_attribute(mat, CD_AUTO_FROM_NAME, "density");
+    GPU_link(mat, "node_attribute_density", out[1].link, &out[1].link);
   }
   if (out[2].hasoutput) {
     out[2].link = GPU_attribute(mat, CD_AUTO_FROM_NAME, "flame");
+    GPU_link(mat, "node_attribute_flame", out[2].link, &out[2].link);
   }
   if (out[3].hasoutput) {
     out[3].link = GPU_attribute(mat, CD_AUTO_FROM_NAME, "temperature");