screen_draw_rect.c

void screen_draw_rect(uint8_t screen[][4], uint8_t x, uint8_t y, uint8_t w, uint8_t h) {
  uint8_t masks[4] = { 0 };

  uint8_t x2 = x + w;
  while (x < x2) {
    uint8_t n = x / 8;
    uint8_t m = x % 8;
    masks[n] |= (1 << (7 - m));
    x++;
  }

  uint8_t y2 = y + h;
  while (y < y2) {
    for (uint8_t i = 0; i < 4; i++) {
      screen[i][y] |= masks[i];
    }
    y++;
  }
}