Add support for 512 palette, new deepmhasis method

- Increase palette table from 256 to 1024. the 1st 256 colors are used
  by FCEUmm internally and for compatibility (PS2/PSP will use the old
  emphasis located in this area). New 512 colors with emphasis will be
  read from offset 256 of this palette table.
- Support loading of 512 palette, this will still use the same nes.pal
  filename.

src/video.c

@@ -61,7 +61,7 @@ int FCEU_InitVirtualVideo(void)
       return 0;
 
    memset(XBuf, 128, 256 * (256 + extrascanlines + 8));
-   memset(XDBuf, 128, 256 * (256 + extrascanlines + 8));
+   memset(XDBuf, 0, 256 * (256 + extrascanlines + 8));
    return 1;
 }