chore(ppu): rename Pixel to Pixels

Since the Pixel struct represents 8 pixels in a 2BPP format, I felt like
the name of the type needed to represent this

src/ppu/registers.rs

@@ -303,9 +303,11 @@ impl From<ObjectPalette> for u8 {
     }
 }
 
-pub struct Pixel(u8, u8);
+pub struct Pixels(u8, u8);
+
+impl Pixels {
+    pub const PIXEL_COUNT: usize = 8;
 
-impl Pixel {
     pub fn from_bytes(higher: u8, lower: u8) -> Self {
         Self(higher, lower)
     }