Add new public var with mapname and new constant (#687)

* Add new public var with current map name

* Add a constant to define max map's name length

* Reducing the maximum buffer size to store a map's name

* The PLATFORM_MAX_PATH constant replaced to MAX_MAPNAME_LENGTH

public/engine_strucs.h

@@ -22,6 +22,7 @@
 #define MAX_LIGHTSTYLES        64
 #define MAX_PACKET_ENTITIES    256
 #define MAX_PATH_LENGTH        260
+#define MAX_MAPNAME_LENGTH	   64
 
 #define MAX_LATENT             32
 #define FRAGMENT_SIZE          1400