Baking: new method to generate margin, based on adjacent faces

This significantly reduces discontinuities on UV seams, by giving a better
match of the texture filtered colors on both sides of the seam. It works by
using pixels from adjacent faces across the UV seam.

This new option is called "Adjacent Faces" and is the default. The old option
is called "Extend", and extends border pixels outwards.

Differential Revision: https://developer.blender.org/D13303

source/blender/blenloader/intern/versioning_legacy.c

@@ -1859,7 +1859,8 @@ void blo_do_versions_pre250(FileData *fd, Library *lib, Main *bmain)
     if (bmain->subversionfile < 4) {
       for (sce = bmain->scenes.first; sce; sce = sce->id.next) {
         sce->r.bake_mode = 1; /* prevent to include render stuff here */
-        sce->r.bake_filter = 16;
+        sce->r.bake_margin = 16;
+        sce->r.bake_margin_type = R_BAKE_ADJACENT_FACES;
         sce->r.bake_flag = R_BAKE_CLEAR;
       }
     }