Fix: EEVEE-Next: Shader compilation warnings in surfel list

False warnings about variable being used before initialization.

source/blender/draw/engines/eevee_next/shaders/eevee_surfel_list_sort_comp.glsl

@@ -110,7 +110,7 @@ void main()
   List unsorted_list = list_split_after(sorted_list, sorted_list.first);
 
   /* Mutable for-each. */
-  for (int i = unsorted_list.first, next; i > -1; i = next) {
+  for (int i = unsorted_list.first, next = 0; i > -1; i = next) {
     next = surfel_buf[i].next;
 
     bool insert = false;
@@ -139,7 +139,7 @@ void main()
    * are not directly perpendicular to the surface. */
 
   /* Mutable foreach. */
-  for (int i = sorted_list.first, next; i > -1; i = next) {
+  for (int i = sorted_list.first, next = 0; i > -1; i = next) {
     next = surfel_buf[i].next;
 
     int valid_next = surfel_buf[i].next;