fn BitDepth::pxstride: Implement generically over stride type, e.…

…x. `isize` or `usize`.

This way we don't have to do `BD::pxstride(stride as usize) as isize`,
which is verbose and wrong for negative strides.
There may still be some places where strides are always positive, though, so it's kept generic.
This also lets us fix #636 gradually; this is the first major step in fixing #636.

include/common/bitdepth.rs

@@ -8,8 +8,11 @@ use std::fmt::Display;
 use std::fmt::Formatter;
 use std::mem;
 use std::ops::Add;
+use std::ops::Div;
 use std::ops::Mul;
+use std::ops::Rem;
 use std::ops::Shr;
+use to_method::To as _;
 
 pub trait FromPrimitive<T> {
     fn from_prim(t: T) -> Self;
@@ -174,9 +177,13 @@ pub trait BitDepth: Clone + Copy {
         clip(pixel, 0.into(), self.bitdepth_max())
     }
 
-    fn pxstride(n: usize) -> usize {
-        let scale = mem::size_of::<Self::Pixel>();
-        debug_assert!(n % scale == 0);
+    /// `T` is generally meant to be `usize` or `isize`.
+    fn pxstride<T>(n: T) -> T
+    where
+        T: Copy + Eq + TryFrom<usize> + From<u8> + Div<Output = T> + Rem<Output = T>,
+    {
+        let scale = mem::size_of::<Self::Pixel>().try_to::<T>().ok().unwrap();
+        debug_assert!(n % scale == 0.into());
         n / scale
     }