Startup.blend: set EEVEE as the default engine

The original idea was to use Clay as the default. But since clay may not
even be in the final release, we can't afford to have a startup.blend
that doesn't deliver a reliable result regardless of build options.

source/blender/blenloader/intern/versioning_defaults.c

@@ -28,6 +28,7 @@
 #include "BLI_utildefines.h"
 #include "BLI_listbase.h"
 #include "BLI_math.h"
+#include "BLI_string.h"
 
 #include "DNA_brush_types.h"
 #include "DNA_freestyle_types.h"
@@ -88,6 +89,8 @@ void BLO_update_defaults_userpref_blend(void)
 void BLO_update_defaults_startup_blend(Main *bmain)
 {
 	for (Scene *scene = bmain->scene.first; scene; scene = scene->id.next) {
+		BLI_strncpy(scene->r.engine, RE_engine_id_BLENDER_EEVEE, sizeof(scene->r.engine));
+
 		scene->r.im_format.planes = R_IMF_PLANES_RGBA;
 		scene->r.im_format.compress = 15;