Tilemap: Use simple "TexPool" replacement for atlas allocation

Releasing a Tilemap atlas into the pool on every map switch
will blow out tons of smaller textures for very little gain,
as atlas textures are already pretty much impossible to
recycle anywhere but in new Tilemaps.

src/sharedstate.h

@@ -123,6 +123,11 @@ struct SharedState
 
 	Quad &gpQuad();
 
+	/* Basically just a simple "TexPool"
+	 * replacement for Tilemap atlas use */
+	void requestAtlasTex(int w, int h, TEXFBO &out);
+	void releaseAtlasTex(TEXFBO &tex);
+
 	/* Checks EventThread's shutdown request flag and if set,
 	 * requests the binding to terminate. In this case, this
 	 * function will most likely not return */