Math: default-constructed Color4 should have zero alpha.

Makes more sense than fully opaque black. On the other hand, creating
Color4 from Color3 or separate RGB components still sets alpha to one,
because that's the intuitive behavior.

src/Magnum/Math/Color.h

@@ -445,10 +445,9 @@ class Color4: public Vector4<T> {
         /**
          * @brief Default constructor
          *
-         * RGB components are set to zero, A component is set to `1.0` for
-         * floating-point types and maximum positive value for integral types.
+         * All components are set to zero.
          */
-        constexpr /*implicit*/ Color4(): Vector4<T>(T(0), T(0), T(0), Implementation::fullChannel<T>()) {}
+        constexpr /*implicit*/ Color4(): Vector4<T>(T(0), T(0), T(0), T(0)) {}
 
         /** @copydoc Vector::Vector(ZeroInitT) */
         constexpr explicit Color4(ZeroInitT)

src/Magnum/Math/Test/ColorTest.cpp

@@ -125,7 +125,7 @@ void ColorTest::constructDefault() {
 
     constexpr Color4 b;
     constexpr Color4ub c;
-    CORRADE_COMPARE(b, Color4(0.0f, 0.0f, 0.0f, 1.0f));
+    CORRADE_COMPARE(b, Color4(0.0f, 0.0f, 0.0f, 0.0f));
     CORRADE_COMPARE(c, Color4ub(0, 0, 0, 255));
 }