Shaders: remove incorrect light direction normalization in Phong.vert.

Adapted slight changes in existing tests, the new ground truth now
passes.

doc/changelog.dox

@@ -145,6 +145,9 @@ See also:
 -   For meshes with multiple sets of vertex attributes (such as texture
     coordinates), @ref MeshTools::compile() should be using only the first set
     but it wasn't.
+-   @ref Shaders::Phong was normalizing light direction in vertex shader,
+    causing the fragment-interpolated direction being incorrect with visible
+    artifacts on long polygons under low light angle
 
 @subsection changelog-latest-deprecated Deprecated APIs
 

src/Magnum/Shaders/Phong.vert

@@ -180,7 +180,7 @@ void main() {
 
     /* Direction to the light */
     for(int i = 0; i < LIGHT_COUNT; ++i)
-        lightDirections[i] = normalize(lightPositions[i] - transformedPosition);
+        lightDirections[i] = lightPositions[i] - transformedPosition;
 
     /* Direction to the camera */
     cameraDirection = -transformedPosition;