EEVEE: Fix sky zenith bug

Careless use of acos() in spherical coordinates transformation was
deteriorating the precision near zenith (and nadir) and producing
glitchy pixels (best seen in longer focal lengths).

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D8266

source/blender/gpu/shaders/material/gpu_shader_material_tex_sky.glsl

@@ -14,7 +14,7 @@ float sky_angle_between(float thetav, float phiv, float theta, float phi)
 
 vec3 sky_spherical_coordinates(vec3 dir)
 {
-  return vec3(acos(dir.z), atan(dir.x, dir.y), 0);
+  return vec3(M_PI_2 - atan(dir.z, length(dir.xy)), atan(dir.x, dir.y), 0);
 }
 
 /* Preetham */