Preallocate vector on reading in file

Provides 1.5% overall speedup on -o4

CHANGELOG.md

@@ -1,6 +1,7 @@
 ### Version 0.19.1 (unreleased)
  - Refactor of internal code.
  - Implement unix-specific permissions copying for `-p` option
+ - Performance optimizations
 
 ### Version 0.19.0
  - [SEMVER_MAJOR] Default to overwriting the input file if `out_file` is not set.

src/png/mod.rs

@@ -73,7 +73,8 @@ impl PngData {
             return Err(PngError::new("Failed to read from file"));
         }
         // Read raw png data into memory
-        let mut byte_data: Vec<u8> = Vec::new();
+        let mut byte_data: Vec<u8> =
+            Vec::with_capacity(file.metadata().map(|m| m.len() as usize).unwrap_or(0));
         match file.read_to_end(&mut byte_data) {
             Ok(_) => (),
             Err(_) => return Err(PngError::new("Failed to read from file")),