filename filtering for media files

No more spaces in filenames. The only allowed chars are a-z, A-Z, 0-9 and
"-_.()".

An exception are texture filenames, which may also have a prefix command
like "<decal>" included.

Also fixed some memory leaks in media download.

source/src/zip.cpp

@@ -211,6 +211,7 @@ bool extractzipfile(ziparchive *a, zipfile *f, const char *name);
 
 bool fitspackage(char *filename, int type)
 {
+    if(strchr(filename, ' ')) return false; // refuse all filenames with spaces
     char *extension = strrchr(filename, '.');
     if(!extension) return false;
     ++extension;