Use XDG_DATA_HOME for Flatpak

Use XDG_DATA_HOME for Flatpak so that fs_homepath is accessible without
special permissions/handling. This changes the home path for Flatpak
from ~/.q3a/ to ~/.var/app/org.ioquake3.ioquake3/data/q3a/.

code/sys/sys_unix.c

@@ -65,24 +65,56 @@ char *Sys_DefaultHomePath(void)
 
 	if( !*homePath && com_homepath != NULL )
 	{
+#ifdef __APPLE__
 		if( ( p = getenv( "HOME" ) ) != NULL )
 		{
 			Com_sprintf(homePath, sizeof(homePath), "%s%c", p, PATH_SEP);
-#ifdef __APPLE__
+
 			Q_strcat(homePath, sizeof(homePath),
 				"Library/Application Support/");
 
 			if(com_homepath->string[0])
 				Q_strcat(homePath, sizeof(homePath), com_homepath->string);
 			else
 				Q_strcat(homePath, sizeof(homePath), HOMEPATH_NAME_MACOSX);
+		}
 #else
+		if( ( p = getenv( "FLATPAK_ID" ) ) != NULL && *p != '\0' )
+		{
+			if( ( p = getenv( "XDG_DATA_HOME" ) ) != NULL && *p != '\0' )
+			{
+				Com_sprintf(homePath, sizeof(homePath), "%s%c", p, PATH_SEP);
+			}
+			else if( ( p = getenv( "HOME" ) ) != NULL && *p != '\0' )
+			{
+				Com_sprintf(homePath, sizeof(homePath), "%s%c.local%cshare%c", p, PATH_SEP, PATH_SEP, PATH_SEP);
+			}
+
+			if( *homePath )
+			{
+				char *dir;
+
+				if(com_homepath->string[0])
+					dir = com_homepath->string;
+				else
+					dir = HOMEPATH_NAME_UNIX;
+
+				if(dir[0] == '.' && dir[1] != '\0')
+					dir++;
+
+				Q_strcat(homePath, sizeof(homePath), dir);
+			}
+		}
+		else if( ( p = getenv( "HOME" ) ) != NULL )
+		{
+			Com_sprintf(homePath, sizeof(homePath), "%s%c", p, PATH_SEP);
+
 			if(com_homepath->string[0])
 				Q_strcat(homePath, sizeof(homePath), com_homepath->string);
 			else
 				Q_strcat(homePath, sizeof(homePath), HOMEPATH_NAME_UNIX);
-#endif
 		}
+#endif
 	}
 
 	return homePath;