VarAction2: Add extra DSG flag for early exit when executing CBs

For when some branches of a non CB switch DSG contain CB switches,
but some don't and so would end up on the graphics chain, as there is
no further CB switch to exit at

src/newgrf_optimiser.cpp

@@ -2647,7 +2647,7 @@ void OptimiseVarAction2DeterministicSpriteGroup(VarAction2OptimiseState &state,
 				}
 			}
 			if (sg != nullptr && sg->type == SGT_CALLBACK) {
-				if (!state.ignore_cb_handler) {
+				if (!state.ignore_cb_handler && static_cast<const CallbackResultSpriteGroup*>(sg)->result != CALLBACK_FAILED) {
 					group->dsg_flags |= DSGF_CB_RESULT;
 					state.have_cb_handler = true;
 				}
@@ -2698,6 +2698,10 @@ void OptimiseVarAction2DeterministicSpriteGroup(VarAction2OptimiseState &state,
 	}
 	if (possible_callback_handler) group->dsg_flags |= DSGF_CB_HANDLER;
 
+	if ((group->dsg_flags & (DSGF_CB_HANDLER | DSGF_CB_RESULT)) == 0) {
+		group->sg_flags |= SGF_SKIP_CB;
+	}
+
 	if (!HasGrfOptimiserFlag(NGOF_NO_OPT_VARACT2_GROUP_PRUNE) && group->ranges.empty() && !group->calculated_result && !seen_req_var1C) {
 		/* There is only one option, remove any redundant adjustments when the result will be ignored anyway */
 		while (!group->adjusts.empty()) {

src/newgrf_spritegroup.cpp

@@ -253,6 +253,11 @@ static bool RangeHighComparator(const DeterministicSpriteGroupRange& range, uint
 
 const SpriteGroup *DeterministicSpriteGroup::Resolve(ResolverObject &object) const
 {
+	if ((this->sg_flags & SGF_SKIP_CB) != 0 && object.callback > 1) {
+		static CallbackResultSpriteGroup cbfail(CALLBACK_FAILED);
+		return &cbfail;
+	}
+
 	uint32 last_value = 0;
 	uint32 value = 0;
 
@@ -615,6 +620,7 @@ void SpriteGroupDumper::DumpSpriteGroup(const SpriteGroup *sg, const char *paddi
 
 	char extra_info[64] = "";
 	if (sg->sg_flags & SGF_ACTION6) strecat(extra_info, " (action 6 modified)", lastof(extra_info));
+	if (sg->sg_flags & SGF_SKIP_CB) strecat(extra_info, " (skip CB)", lastof(extra_info));
 	if (HasBit(_misc_debug_flags, MDF_NEWGRF_SG_DUMP_MORE_DETAIL)) {
 		if (sg->sg_flags & SGF_INLINING) strecat(extra_info, " (inlining)", lastof(extra_info));
 	}

src/newgrf_spritegroup.h

@@ -63,6 +63,7 @@ enum SpriteGroupFlags : uint8 {
 	SGF_NONE                     = 0,
 	SGF_ACTION6                  = 1 << 0,
 	SGF_INLINING                 = 1 << 1,
+	SGF_SKIP_CB                  = 1 << 2,
 };
 DECLARE_ENUM_AS_BIT_SET(SpriteGroupFlags)