Make linearstep definitely return 1 for x > edge1

The division (xclamped - edge0) / (edge1 - edge0) is not guaranteed
to be exactly 1 for xclamped == edge1 if the backend floating point
math has higher than expected error bounds. Eg. it is common on GPU
backends to use faster but less precise instructions for division.

The modified code is mathematically equivalent and it logically
guarantees the returned value is exactly 1.0 for x > edge1 > edge0.

Author: roelandschoukens

src/shaders/stdosl.h

@@ -337,8 +337,7 @@ vector smoothstep (vector edge0, vector edge1, vector x)
 float linearstep (float edge0, float edge1, float x) {
     float result;
     if (edge0 != edge1) {
-        float xclamped = clamp (x, edge0, edge1);
-        result = (xclamped - edge0) / (edge1 - edge0);
+        result = clamp((x - edge0) / (edge1 - edge0), 0, 1);
     } else {  // special case: edges coincide
         result = step (edge0, x);
     }