Show (short) list of possible enum values when an invalid value is encountered (resolves #1318)

All enum errors have (hopefully) been changed to a luax_enumerror, which has a
fixed error message. If additionally a list of valid options is passed, it
lists that in the error message. For every enum error with few options I've
implemented this using a getConstants call.

This solution has been designed specifically to reduce the number of template
instantiations (as I've been told that was a concern). All new template code
happens in places where there already was an instantiation of the relevant
StringMap. Of course there is still std::vector<std::string>...

--HG--
branch : minor

src/modules/graphics/depthstencil.h

@@ -20,6 +20,9 @@
 
 #pragma once
 
+#include <vector>
+#include <string>
+
 namespace love
 {
 namespace graphics
@@ -60,9 +63,11 @@ CompareMode getReversedCompareMode(CompareMode mode);
 
 bool getConstant(const char *in, StencilAction &out);
 bool getConstant(StencilAction in, const char *&out);
+std::vector<std::string> getConstants(StencilAction);
 
 bool getConstant(const char *in, CompareMode &out);
 bool getConstant(CompareMode in, const char *&out);
+std::vector<std::string> getConstants(CompareMode);
 
 } // graphics
 } // love