PNG loading doesn't support v8 files
PNG loading doesn't support v8 files

And maybe other versions?

```
Matts-iMac:picolove matt$ love . 26413.p8.png 
Loading /26413.p8.png
version 8
codelen 27094
Error: main.lua:598: unknown file version 8
stack traceback:
    [C]: in function 'error'
    main.lua:598: in function 'load_p8'
    main.lua:1705: in function '_load'
    main.lua:352: in function 'load'
    main.lua:888: in function <main.lua:878>
    [C]: in function 'xpcall'
```
