Remove num_cpus crate

Since Rust 1.59.0, we can use `std::thread::available_parallelism`
to get "an estimate of default amount of parallelism". Since sometimes
the number of CPU is NOT same as *available parallelism*. it's better
to use this function instead of `num_cpus::get()` function.

Related to: #78 (see changelog)

Cargo.toml

@@ -12,7 +12,6 @@ glob = "^0.3"
 image = "^0.24"
 log = "^0.4"
 minifb = "^0.25"
-num_cpus = "^1.16"
 rand = "^0.8"
 rayon = "^1.8"
 simple_logger = "^4.2"

src/main.rs

@@ -210,8 +210,9 @@ fn image_buffer_from_filepath<P>(
         let (width, height) = rgb.dimensions();
         let mut buf: Vec<u32> = vec![0; (width * height) as usize];
 
-        let threads = num_cpus::get();
-        let rows_per_band = height / threads as u32 + 1;
+        let threads =
+            std::thread::available_parallelism().expect("Failed to get available parallelism num");
+        let rows_per_band = height / threads.get() as u32 + 1;
         let bands: Vec<&mut [u32]> = buf.chunks_mut((rows_per_band * width) as usize).collect();
         bands.into_par_iter().enumerate().for_each(|(i, band)| {
             for (j, b) in band.iter_mut().enumerate() {