reduce low-level byte fiddling in openexr encoder (#2012)

* use more high-level function `pod_read_unaligned` to read [f32;3] array instead of manually constructing it component by component

* reformat with rustfmt

* beautify (apply clippy suggestions)

src/codecs/openexr.rs

@@ -28,7 +28,6 @@ use crate::{
     ColorType, ExtendedColorType, ImageDecoder, ImageEncoder, ImageError, ImageFormat, ImageResult,
     Progress,
 };
-use std::convert::TryInto;
 use std::io::{Cursor, Read, Seek, Write};
 
 /// An OpenEXR decoder. Immediately reads the meta data from the file.
@@ -272,27 +271,22 @@ fn write_buffer(
         }
     }
 
-    // bytes might be unaligned so we cannot cast the whole thing, instead lookup each f32 individually
-    let lookup_f32 = move |f32_index: usize| {
-        let unaligned_f32_bytes_slice = &unaligned_bytes[f32_index * 4..(f32_index + 1) * 4];
-        let f32_bytes_array = unaligned_f32_bytes_slice
-            .try_into()
-            .expect("indexing error");
-        f32::from_ne_bytes(f32_bytes_array)
-    };
+    let bytes_per_pixel = color_type.bytes_per_pixel() as usize;
 
     match color_type {
         ColorType::Rgb32F => {
             exr::prelude::Image // TODO compression method zip??
                 ::from_channels(
                 (width, height),
                 SpecificChannels::rgb(|pixel: Vec2<usize>| {
-                    let pixel_index = 3 * pixel.flat_index_for_size(Vec2(width, height));
-                    (
-                        lookup_f32(pixel_index),
-                        lookup_f32(pixel_index + 1),
-                        lookup_f32(pixel_index + 2),
-                    )
+                    let pixel_index = pixel.flat_index_for_size(Vec2(width, height));
+                    let start_byte = pixel_index * bytes_per_pixel;
+
+                    let [r, g, b]: [f32; 3] = bytemuck::pod_read_unaligned(
+                        &unaligned_bytes[start_byte..start_byte + bytes_per_pixel],
+                    );
+
+                    (r, g, b)
                 }),
             )
             .write()
@@ -306,13 +300,14 @@ fn write_buffer(
                 ::from_channels(
                 (width, height),
                 SpecificChannels::rgba(|pixel: Vec2<usize>| {
-                    let pixel_index = 4 * pixel.flat_index_for_size(Vec2(width, height));
-                    (
-                        lookup_f32(pixel_index),
-                        lookup_f32(pixel_index + 1),
-                        lookup_f32(pixel_index + 2),
-                        lookup_f32(pixel_index + 3),
-                    )
+                    let pixel_index = pixel.flat_index_for_size(Vec2(width, height));
+                    let start_byte = pixel_index * bytes_per_pixel;
+
+                    let [r, g, b, a]: [f32; 4] = bytemuck::pod_read_unaligned(
+                        &unaligned_bytes[start_byte..start_byte + bytes_per_pixel],
+                    );
+
+                    (r, g, b, a)
                 }),
             )
             .write()