Fix T42391: HSV correction shader node gives negative values

This mainly happens when over-saturating already saturated color.
After some discussion with Campbell and loads of tests we decided
to clamp the result RGB color. As an alternative we might want to
clamp corrected HSV values instead, but that would lead to some
larger changes in the render results.

TODO: The same is to be done for compositor nodes.

intern/cycles/kernel/shaders/node_hsv.osl

@@ -35,6 +35,11 @@ shader node_hsv(
 
 	Color = hsv_to_rgb(Color);
 
+	// Clamp color to prevent negative values cauzed by oversaturation.
+	Color[0] = max(Color[0], 0.0);
+	Color[1] = max(Color[1], 0.0);
+	Color[2] = max(Color[2], 0.0);
+
 	ColorOut = mix(ColorIn, Color, Fac);
 }
 

intern/cycles/kernel/svm/svm_hsv.h

@@ -46,6 +46,11 @@ ccl_device void svm_node_hsv(KernelGlobals *kg, ShaderData *sd, float *stack, ui
 	color.y = fac*color.y + (1.0f - fac)*in_color.y;
 	color.z = fac*color.z + (1.0f - fac)*in_color.z;
 
+	/* Clamp color to prevent negative values cauzed by oversaturation. */
+	color.x = max(color.x, 0.0f);
+	color.y = max(color.y, 0.0f);
+	color.z = max(color.z, 0.0f);
+
 	if (stack_valid(out_color_offset))
 		stack_store_float3(stack, out_color_offset, color);
 }