Fix really bad bug with SIMD usage in the Sobol sampler.

Wasn't using the bit shifting instructions the correct way.  SSE
instructions are very non-obvious...

sub_crates/sobol/src/wide.rs

@@ -22,6 +22,11 @@ pub(crate) mod sse {
             }
         }
 
+        pub fn get(self, i: usize) -> u32 {
+            let n: [u32; 4] = unsafe { std::mem::transmute(self) };
+            n[i]
+        }
+
         /// Converts the full range of a 32 bit integer to a float in [0, 1).
         #[inline(always)]
         pub fn to_norm_floats(self) -> [f32; 4] {
@@ -203,7 +208,7 @@ pub(crate) mod sse {
         #[inline(always)]
         fn shl(self, other: i32) -> Int4 {
             Int4 {
-                v: unsafe { _mm_sll_epi32(self.v, _mm_set1_epi32(other)) },
+                v: unsafe { _mm_sll_epi32(self.v, _mm_set_epi32(0, 0, 0, other)) },
             }
         }
     }
@@ -214,7 +219,7 @@ pub(crate) mod sse {
         #[inline(always)]
         fn shr(self, other: i32) -> Int4 {
             Int4 {
-                v: unsafe { _mm_srl_epi32(self.v, _mm_set1_epi32(other)) },
+                v: unsafe { _mm_srl_epi32(self.v, _mm_set_epi32(0, 0, 0, other)) },
             }
         }
     }
@@ -227,6 +232,38 @@ pub(crate) mod sse {
             }
         }
     }
+
+    #[cfg(test)]
+    mod tests {
+        use super::*;
+
+        #[test]
+        fn from_array() {
+            let a = Int4::from([1, 2, 3, 4]);
+            assert_eq!(a.get(0), 1);
+            assert_eq!(a.get(1), 2);
+            assert_eq!(a.get(2), 3);
+            assert_eq!(a.get(3), 4);
+        }
+
+        #[test]
+        fn shr() {
+            let a = Int4::from([0xffffffff; 4]) >> 16;
+            assert_eq!(a.get(0), 0x0000ffff);
+            assert_eq!(a.get(1), 0x0000ffff);
+            assert_eq!(a.get(2), 0x0000ffff);
+            assert_eq!(a.get(3), 0x0000ffff);
+        }
+
+        #[test]
+        fn shl() {
+            let a = Int4::from([0xffffffff; 4]) << 16;
+            assert_eq!(a.get(0), 0xffff0000);
+            assert_eq!(a.get(1), 0xffff0000);
+            assert_eq!(a.get(2), 0xffff0000);
+            assert_eq!(a.get(3), 0xffff0000);
+        }
+    }
 }
 #[cfg(target_arch = "x86_64")]
 pub(crate) use sse::Int4;