Shaders: clamp shininess factor in Phong to minimize artifacts.

I feel this makes the shader code unnecessarily slow to handle bad
parameters. But eh, until I actually benchmark, I shouldn't be making
assumptions.

doc/changelog.dox

@@ -292,6 +292,11 @@ See also:
     also with @ref Corrade::Containers::ArrayView,
     @ref Corrade::Containers::StridedArrayView and friends
 
+@subsubsection changelog-latest-changes-shaders Shaders library
+
+-   @ref Shaders::Phong now clamps the specular factor to minimize artifacts
+    when shininess is near zero
+
 @subsubsection changelog-latest-changes-texturetools TextureTools library
 
 -   @ref TextureTools::distanceField() was updated to work on ES3 SwiftShader

src/Magnum/Shaders/Phong.frag

@@ -181,7 +181,7 @@ void main() {
         /* Add specular color, if needed */
         if(intensity > 0.001) {
             highp vec3 reflection = reflect(-normalizedLightDirection, normalizedTransformedNormal);
-            mediump float specularity = pow(max(0.0, dot(normalize(cameraDirection), reflection)), shininess);
+            mediump float specularity = clamp(pow(max(0.0, dot(normalize(cameraDirection), reflection)), shininess), 0.0, 1.0);
             color += vec4(finalSpecularColor.rgb*specularity, finalSpecularColor.a);
         }
     }