Subdiv: Use better default UV interpolation

The default one is now UVs are smoothed, boundaries are kept sharp.

After some time of experimentation seems this is better default from
interoperability point of view.

This fixes distortion reported in T83470.

Differential Revision: https://developer.blender.org/D10111

source/blender/blenkernel/intern/multires.c

@@ -781,7 +781,7 @@ static DerivedMesh *subsurf_dm_create_local(Scene *scene,
   smd.levels = smd.renderLevels = lvl;
   smd.quality = 3;
   if (!is_plain_uv) {
-    smd.uv_smooth = SUBSURF_UV_SMOOTH_PRESERVE_CORNERS;
+    smd.uv_smooth = SUBSURF_UV_SMOOTH_PRESERVE_BOUNDARIES;
   }
   else {
     smd.uv_smooth = SUBSURF_UV_SMOOTH_NONE;

source/blender/makesdna/DNA_modifier_defaults.h

@@ -440,7 +440,7 @@
     .renderlvl = 0, \
     .totlvl = 0, \
     .flags = eMultiresModifierFlag_UseCrease | eMultiresModifierFlag_ControlEdges, \
-    .uv_smooth = SUBSURF_UV_SMOOTH_PRESERVE_CORNERS, \
+    .uv_smooth = SUBSURF_UV_SMOOTH_PRESERVE_BOUNDARIES, \
     .quality = 4, \
     .boundary_smooth = SUBSURF_BOUNDARY_SMOOTH_ALL, \
   }
@@ -622,7 +622,7 @@
     .levels = 1, \
     .renderLevels = 2, \
     .flags = eSubsurfModifierFlag_UseCrease | eSubsurfModifierFlag_ControlEdges, \
-    .uv_smooth = SUBSURF_UV_SMOOTH_PRESERVE_CORNERS, \
+    .uv_smooth = SUBSURF_UV_SMOOTH_PRESERVE_BOUNDARIES, \
     .quality = 3, \
     .boundary_smooth = SUBSURF_BOUNDARY_SMOOTH_ALL, \
     .emCache = NULL, \

source/blender/render/intern/multires_bake.c

@@ -768,7 +768,7 @@ static void *init_heights_data(MultiresBakeRender *bkr, Image *ima)
 
     if (ss_lvl > 0) {
       smd.levels = smd.renderLevels = ss_lvl;
-      smd.uv_smooth = SUBSURF_UV_SMOOTH_PRESERVE_CORNERS;
+      smd.uv_smooth = SUBSURF_UV_SMOOTH_PRESERVE_BOUNDARIES;
       smd.quality = 3;
 
       height_data->ssdm = subsurf_make_derived_from_derived(