(svn r10143) -Add: store the filename of the grfs opened and allow easy access to the name

-Codechange: store the SpriteID in the spritecache too
-Add: add a PNG loader for graphical files
-Documentation: added a document to explain the PNG format

src/fileio.h

@@ -8,6 +8,7 @@
 void FioSeekTo(uint32 pos, int mode);
 void FioSeekToFile(uint32 pos);
 uint32 FioGetPos();
+const char *FioGetFilename();
 byte FioReadByte();
 uint16 FioReadWord();
 uint32 FioReadDword();