BLI: fix math operation

That fixes a stupid mistake of mine that was copied a couple of times.

source/blender/blenlib/BLI_double2.hh

@@ -120,7 +120,8 @@ struct double2 {
 
   static double distance_squared(const double2 &a, const double2 &b)
   {
-    return double2::dot(a, b);
+    double2 diff = a - b;
+    return double2::dot(diff, diff);
   }
 
   struct isect_result {

source/blender/blenlib/BLI_double3.hh

@@ -218,7 +218,8 @@ struct double3 {
 
   static double distance_squared(const double3 &a, const double3 &b)
   {
-    return double3::dot(a, b);
+    double3 diff = a - b;
+    return double3::dot(diff, diff);
   }
 
   static double3 interpolate(const double3 &a, const double3 &b, double t)

source/blender/blenlib/BLI_float2.hh

@@ -141,7 +141,8 @@ struct float2 {
 
   static float distance_squared(const float2 &a, const float2 &b)
   {
-    return float2::dot(a, b);
+    float2 diff = a - b;
+    return float2::dot(diff, diff);
   }
 
   struct isect_result {

source/blender/blenlib/BLI_float3.hh

@@ -236,7 +236,8 @@ struct float3 {
 
   static float distance_squared(const float3 &a, const float3 &b)
   {
-    return float3::dot(a, b);
+    float3 diff = a - b;
+    return float3::dot(diff, diff);
   }
 
   static float3 interpolate(const float3 &a, const float3 &b, float t)

source/blender/blenlib/BLI_mpq2.hh

@@ -156,7 +156,8 @@ struct mpq2 {
 
   static mpq_class distance_squared(const mpq2 &a, const mpq2 &b)
   {
-    return dot(a, b);
+    mpq2 diff = a - b;
+    return dot(diff, diff);
   }
 
   struct isect_result {