Add a "moving sprite" sample

[jonathanpeppers]

It moves via the D pad.

Equivalent C program:

#include <stdlib.h>
#include <string.h>
#include "neslib.h"

//#link "chr_generic.s"

const char PALETTE[32] = {
  0x30,               // screen color

  0x11,0x30,0x27,0x0, // background palette 0
  0x1c,0x20,0x2c,0x0, // background palette 1
  0x00,0x10,0x20,0x0, // background palette 2
  0x06,0x16,0x26,0x0, // background palette 3

  0x14,0x34,0x0d,0x0, // sprite palette 0
  0x00,0x37,0x25,0x0, // sprite palette 1
  0x0d,0x2d,0x3a,0x0, // sprite palette 2
  0x0d,0x27,0x2a      // sprite palette 3
};

void main() {
  char x = 40;
  char y = 40;
  char pad;

  pal_all(PALETTE);
  ppu_on_all();

  while (1) {
    ppu_wait_nmi();

    pad = pad_poll(0);

    if (pad & PAD_LEFT)  x--;
    if (pad & PAD_RIGHT) x++;
    if (pad & PAD_UP)    y--;
    if (pad & PAD_DOWN)  y++;

    // Draw 2x2 sprite (16x16 pixels)
    oam_spr(x,     y,     0xD8, 0, 0);
    oam_spr(x + 8, y,     0xDA, 0, 4);
    oam_spr(x,     y + 8, 0xD9, 0, 8);
    oam_spr(x + 8, y + 8, 0xDB, 0, 12);
  }
}