Fix nishita sky artifacts on some GPUs

It could happen that values returned by dot() were slightly larger than 1 or less than -1 due to precision errors, but acos() is undefined outside of [-1, 1]. This would lead to NaN values on some GPUs, causing visible artifacts.

Shaders/std/math.glsl

@@ -40,4 +40,9 @@ float attenuate(const float dist) {
 	// 1.0 / (quadratic * dist * dist);
 }
 
+float safe_acos(const float x) {
+	// acos is undefined if |x| > 1
+	return acos(clamp(x, -1.0, 1.0));
+}
+
 #endif

Shaders/std/sky.glsl

@@ -20,6 +20,8 @@
 #ifndef _SKY_GLSL_
 #define _SKY_GLSL_
 
+#include "std/math.glsl"
+
 uniform sampler2D nishitaLUT;
 uniform vec2 nishitaDensity;
 
@@ -119,7 +121,7 @@ vec3 nishita_atmosphere(const vec3 r, const vec3 r0, const vec3 pSun, const floa
 
 		// Idea behind this: "Rotate" everything by iPos (-> iPos is the new zenith) and then all calculations for the
 		// inner integral only depend on the sample height (iHeight) and sunTheta (angle between sun and new zenith).
-		float sunTheta = acos(dot(normalize(iPos), normalize(pSun)));
+		float sunTheta = safe_acos(dot(normalize(iPos), normalize(pSun)));
 		vec3 jAttn = nishita_lookupLUT(iHeight, sunTheta);
 
 		// Calculate attenuation