Query SDL for available texture renderers

There's no point in hardcoding these values - they can be different to
every user (it depends on hardware, OS, SDL version, etc).

Also, future versions of SDL might introduce more renderers, so this way
the code is more future-proof.

include/setup.h

@@ -123,6 +123,7 @@ public:
 
 	Property(std::string const& _propname, Changeable::Value when):propname(_propname),change(when) { }
 	void Set_values(const char * const * in);
+	void Set_values(const std::vector<std::string> &in);
 	void Set_help(std::string const& str);
 	char const* Get_help();
 	virtual	bool SetValue(std::string const& str)=0;