Cleanup: Use interpf instead of repeating the logic

This makes the code clearer.

source/blender/editors/transform/transform_mode_curveshrinkfatten.c

@@ -64,7 +64,7 @@ static void applyCurveShrinkFatten(TransInfo *t, const int UNUSED(mval[2]))
       if (td->val) {
         *td->val = td->ival * ratio;
         /* apply PET */
-        *td->val = (*td->val * td->factor) + ((1.0f - td->factor) * td->ival);
+        *td->val = interpf(*td->val, td->ival, td->factor);
         CLAMP_MIN(*td->val, 0.0f);
       }
     }

source/blender/editors/transform/transform_mode_gpopacity.c

@@ -74,7 +74,7 @@ static void applyGPOpacity(TransInfo *t, const int UNUSED(mval[2]))
       if (td->val) {
         *td->val = td->ival * ratio;
         /* apply PET */
-        *td->val = (*td->val * td->factor) + ((1.0f - td->factor) * td->ival);
+        *td->val = interpf(*td->val, td->ival, td->factor);
         CLAMP(*td->val, 0.0f, 1.0f);
       }
     }

source/blender/editors/transform/transform_mode_gpshrinkfatten.c

@@ -74,7 +74,7 @@ static void applyGPShrinkFatten(TransInfo *t, const int UNUSED(mval[2]))
       if (td->val) {
         *td->val = td->ival * ratio;
         /* apply PET */
-        *td->val = (*td->val * td->factor) + ((1.0f - td->factor) * td->ival);
+        *td->val = interpf(*td->val, td->ival, td->factor);
         if (*td->val <= 0.0f) {
           *td->val = 0.001f;
         }

source/blender/editors/transform/transform_mode_maskshrinkfatten.c

@@ -90,7 +90,7 @@ static void applyMaskShrinkFatten(TransInfo *t, const int UNUSED(mval[2]))
         }
 
         /* apply PET */
-        *td->val = (*td->val * td->factor) + ((1.0f - td->factor) * td->ival);
+        *td->val = interpf(*td->val, td->ival, td->factor);
         if (*td->val <= 0.0f) {
           *td->val = 0.001f;
         }