chore(ppu): rename select structs

src/ppu/registers.rs

@@ -303,9 +303,9 @@ impl From<ObjectPalette> for u8 {
     }
 }
 
-pub struct TwoBitsPerPixel(u8, u8);
+pub struct Pixel(u8, u8);
 
-impl TwoBitsPerPixel {
+impl Pixel {
     pub fn from_bytes(higher: u8, lower: u8) -> Self {
         Self(higher, lower)
     }