Fix T96401: Broken multires baked normals result

A 7 year old commit, 2ec00ea0, used incorrect indexing for
the optional array of precomputed poly normals. Apparently that code
path was never used, or this issue would have been discovered earlier.
Recent changes calculate normals on a temporary mesh and use those
for the "low-res" layer, meaning the precomputed path was always taken.

source/blender/render/intern/multires_bake.c

@@ -82,7 +82,7 @@ typedef struct {
   MLoopUV *mloopuv;
   const MLoopTri *mlooptri;
   float *pvtangent;
-  const float *precomputed_normals;
+  const float (*precomputed_normals)[3];
   int w, h;
   int tri_index;
   DerivedMesh *lores_dm, *hires_dm;
@@ -132,7 +132,7 @@ static void multiresbake_get_normal(const MResolvePixelData *data,
 
   if (!smoothnormal) { /* flat */
     if (data->precomputed_normals) {
-      copy_v3_v3(norm, &data->precomputed_normals[poly_index]);
+      copy_v3_v3(norm, data->precomputed_normals[poly_index]);
     }
     else {
       BKE_mesh_calc_poly_normal(mp, &data->mloop[mp->loopstart], data->mvert, norm);
@@ -558,7 +558,7 @@ static void do_multires_bake(MultiresBakeRender *bkr,
       handle->data.mlooptri = mlooptri;
       handle->data.mloop = mloop;
       handle->data.pvtangent = pvtangent;
-      handle->data.precomputed_normals = (float *)poly_normals; /* don't strictly need this */
+      handle->data.precomputed_normals = poly_normals; /* don't strictly need this */
       handle->data.w = ibuf->x;
       handle->data.h = ibuf->y;
       handle->data.lores_dm = dm;