Fixed "error parsing image modifications" message...

...caused by subsequent image mod additions using the add attribute in
[effect] apply_to=image_mod

changelog

@@ -51,6 +51,8 @@ Version 1.9.9+svn:
    * [illuminated_time], which has been obsolete for a long time, is no longer
      valid. Using it will cause errors to be thrown
    * Reintroduced support for [unit][event]s (was until 1.7.10, bug #16259)
+   * Fixed "error parsing image modifications" message caused by subsequent image
+     mod additions using the add attribute in [effect] apply_to=image_mod
  * Miscellaneous and bug fixes:
    * Fixed compilation on all Debian architectures (Debian bug #636193)
    * Fixed handling of #ifver and #ifnver preprocessor directives in wmllint

data/scenario-test.cfg

@@ -568,6 +568,10 @@ end
                     apply_to=image_mod
                     add="PAL(FFDC02,EA9752,C4793A,C35029,6B2C29,A34C29,D5561F,343424,353624,BF5746 > BCBCBC,7D7D7D,5E5E5E,434343,232323,3B3B3B,494949,232323,232323,484848)"
                 [/effect]
+                [effect]
+                    apply_to=image_mod
+                    add="B(50)~R(-50)"
+                [/effect]
             [/object]
             [attack]
             [/attack]

src/unit.cpp

@@ -2522,6 +2522,10 @@ void unit::add_modification(const std::string& type, const config& mod, bool no_
 					LOG_UT << "applying image_mod \n";
 					mod = effect["add"].str();
 					if (!mod.empty()){
+						if(!image_mods_.empty()) {
+							image_mods_ += '~';
+						}
+
 						image_mods_ += mod;
 					}