Make dependency on image optional

After PR https://github.com/shssoichiro/oxipng/pull/481 was merged, the
`image` dependency became unused when building with debug assertions
disabled, as it is only used to implement output sanity checks when such
assertions are enabled.

The `image` crate transitively pulls a significant amount of
dependencies, so it's useful for OxiPNG users to get rid of them when
not needed.

[Cargo does not allow specifying dependencies that are only pulled when
debug assertions are
enabled](https://github.com/rust-lang/cargo/issues/7634), so the next
best way to give users some flexibility is to gate those debug
assertions behind a feature flag.

These changes add a `sanity-checks` feature flag that controls whether
the `image` crate and the related sanity checks are compiled in. This
feature is enabled by default to keep debug builds useful to catch
problems during development.

Cargo.toml

@@ -53,6 +53,7 @@ optional = true
 version = "2.1.0"
 
 [dependencies.image]
+optional = true
 default-features = false
 features = ["png"]
 version = "0.24.3"
@@ -65,6 +66,7 @@ binary = ["clap", "wild", "stderrlog"]
 default = ["binary", "filetime", "parallel", "zopfli"]
 parallel = ["rayon", "indexmap/rayon", "crossbeam-channel"]
 freestanding = ["libdeflater/freestanding"]
+sanity-checks = ["image"]
 
 [lib]
 name = "oxipng"

src/lib.rs

@@ -31,7 +31,6 @@ use crate::evaluate::Evaluator;
 use crate::png::PngData;
 use crate::png::PngImage;
 use crate::reduction::*;
-use image::{DynamicImage, GenericImageView, ImageFormat, Pixel};
 use log::{debug, error, info, warn};
 use rayon::prelude::*;
 use std::fmt;
@@ -659,7 +658,8 @@ fn optimize_png(
         );
     }
 
-    debug_assert!(validate_output(&output, original_data));
+    #[cfg(feature = "sanity-checks")]
+    debug_assert!(sanity_checks::validate_output(&output, original_data));
 
     Ok(output)
 }
@@ -1044,8 +1044,13 @@ fn copy_times(input_path_meta: &Metadata, out_path: &Path) -> PngResult<()> {
     })
 }
 
+#[cfg(feature = "sanity-checks")]
+mod sanity_checks {
+    use super::*;
+    use image::{DynamicImage, GenericImageView, ImageFormat, Pixel};
+
     /// Validate that the output png data still matches the original image
-fn validate_output(output: &[u8], original_data: &[u8]) -> bool {
+    pub(super) fn validate_output(output: &[u8], original_data: &[u8]) -> bool {
         let (old_png, new_png) = rayon::join(
             || load_png_image_from_memory(original_data),
             || load_png_image_from_memory(output),
@@ -1087,3 +1092,4 @@ fn images_equal(old_png: &DynamicImage, new_png: &DynamicImage) -> bool {
         });
         a.eq(b)
     }
+}