Shaders: default ambient color in Phong to transparent black.

Makes it possible to support alpha-masked drawing out of the box,
shouldn't have any negative effect on anything else.

doc/changelog.dox

@@ -165,6 +165,9 @@ See also:
     @ref Shaders::Phong::bindTextures() now assert that the shader was created
     with the corresponding flag enabled to prevent accidental "black screen of
     death" errors.
+-   Ambient color in untextured @ref Shaders::Phong now defaults to
+    @cpp 0x00000000_rgbaf @ce in order to support alpha-masked drawing out of
+    the box
 
 @subsubsection changelog-latest-changes-texturetools TextureTools library
 
@@ -265,6 +268,10 @@ See also:
     with the corresponding flag enabled to prevent accidental "black screen of
     death" errors. This might cause your application to abort if it was calling
     these functions when not needed.
+-   Ambient color in untextured @ref Shaders::Phong now defaults to
+    @cpp 0x00000000_rgbaf @ce in order to support alpha-masked drawing out of
+    the box. This may break code that assumed the alpha channel being opaque by
+    default.
 -   @ref Trade::PhongMaterialData::ambientColor(),
     @ref Trade::PhongMaterialData::diffuseColor() "diffuseColor()" and
     @ref Trade::PhongMaterialData::specularColor() "specularColor()" now return

src/Magnum/Shaders/Phong.cpp

@@ -117,7 +117,7 @@ Phong::Phong(const Flags flags): _flags(flags) {
     #ifdef MAGNUM_TARGET_GLES
     /* Default to fully opaque white so we can see the textures */
     if(flags & Flag::AmbientTexture) setAmbientColor(Color4{1.0f});
-    else setAmbientColor(Color4{0.0f, 1.0f});
+    else setAmbientColor(Color4{0.0f});
     setDiffuseColor(Color4{1.0f});
     setSpecularColor(Color4{1.0f});
     setLightColor(Color4{1.0f});

src/Magnum/Shaders/Phong.frag

@@ -46,7 +46,7 @@ layout(location = 4)
 uniform lowp vec4 ambientColor
     #ifndef GL_ES
     #ifndef AMBIENT_TEXTURE
-    = vec4(0.0, 0.0, 0.0, 1.0)
+    = vec4(0.0)
     #else
     = vec4(1.0)
     #endif

src/Magnum/Shaders/Phong.h

@@ -209,7 +209,7 @@ class MAGNUM_SHADERS_EXPORT Phong: public GL::AbstractShaderProgram {
          *
          * If @ref Flag::AmbientTexture is set, default value is
          * @cpp 0xffffffff_rgbaf @ce and the color will be multiplied with
-         * ambient texture, otherwise default value is @cpp 0x000000ff_rgbaf @ce.
+         * ambient texture, otherwise default value is @cpp 0x00000000_rgbaf @ce.
          * @see @ref bindAmbientTexture()
          */
         Phong& setAmbientColor(const Color4& color) {