Fix T77298: Can't bake texture with multiple objects

The problem here is that the baking code uses tiles to exchange pixel data with
the renderer since a recent-ish refactor, but the code that sent data to the
renderer did not initialize the bake result pixels.

Therefore, when the baking process for the second object started, Cycles
received empty tiles and sent them back as-is if the second object did not
cover them.

By initializing the tiles with the result of the previous bakes, we avoid this
problem.

source/blender/render/intern/source/external_engine.c

@@ -174,7 +174,8 @@ static RenderResult *render_result_from_bake(RenderEngine *engine, int x, int y,
   BLI_addtail(&rr->layers, rl);
 
   /* Add render passes. */
-  render_layer_add_pass(rr, rl, engine->bake.depth, RE_PASSNAME_COMBINED, "", "RGBA");
+  RenderPass *result_pass = render_layer_add_pass(
+      rr, rl, engine->bake.depth, RE_PASSNAME_COMBINED, "", "RGBA");
   RenderPass *primitive_pass = render_layer_add_pass(rr, rl, 4, "BakePrimitive", "", "RGBA");
   RenderPass *differential_pass = render_layer_add_pass(rr, rl, 4, "BakeDifferential", "", "RGBA");
 
@@ -210,6 +211,15 @@ static RenderResult *render_result_from_bake(RenderEngine *engine, int x, int y,
     }
   }
 
+  /* Initialize tile render result from full image bake result. */
+  for (int ty = 0; ty < h; ty++) {
+    size_t offset = ty * w * engine->bake.depth;
+    size_t bake_offset = ((y + ty) * engine->bake.width + x) * engine->bake.depth;
+    size_t size = w * engine->bake.depth * sizeof(float);
+
+    memcpy(result_pass->rect + offset, engine->bake.result + bake_offset, size);
+  }
+
   return rr;
 }