fix going beyond end of useful data in rgbas array

It is possible for rgbas to be an array with bigger allocated size, with garbage data in the end - make sure to only take the useful data.

src/frame/mod.rs

@@ -9,6 +9,7 @@ pub fn compute_perceived_lightness_percent(rgbas: &[u8], has_alpha: bool, pixels
 
     let (rs, gs, bs) = rgbas
         .iter()
+        .take(channels * pixels)
         .chunks(channels)
         .into_iter()
         .map(|mut chunk| {