Handle the case when surface normal looks away from the camera

This can happen on very bumpy surfaces that are simulated only by normal
maps. In real life these points should be hidden by the relief of the
surface, but in render they are often visible.

They shouldn't be rendered as shaded as it's done for the light-normal
angle, because they can still be lit, but the angle should be clamped
from below.

Author: 10110111

data/shaders/planet.frag

@@ -139,6 +139,8 @@ mediump float orenNayar(in mediump vec3 normal, in highp vec3 lightDir, in highp
     mediump float cosAngleLightNormal = dot(normal, lightDir);  //cos theta_i
     mediump float cosAngleEyeNormal = dot(normal, viewDir); //cos theta_r
     if(cosAngleLightNormal < 0.) return 0.;
+    if(cosAngleEyeNormal < 0.)
+        cosAngleEyeNormal = 0.;
     //acos can be quite expensive, can we avoid it?
     mediump float angleLightNormal = acos(cosAngleLightNormal); //theta_i
     mediump float angleEyeNormal = acos(cosAngleEyeNormal); //theta_r