Math: make _rgbf and _rgbaf literals constexpr.

Need them for the UI. Will eventually need the sRGB literals too, not
sure what to do there yet, std::pow() is only constexpr in C++26. I'll
be probably long retired when *that* version becomes the min spec for
Magnum.

src/Magnum/Math/Color.h

@@ -1393,7 +1393,8 @@ constexpr Vector4<UnsignedByte> operator "" _srgba(unsigned long long value) {
 /** @relatesalso Magnum::Math::Color3
 @brief Float linear RGB literal
 
-Calls @ref Color3::fromLinearRgbInt() on the literal value. Example usage:
+Equivalent to calling @ref Color3::fromLinearRgbInt() on the literal value.
+Example usage:
 
 @snippet MagnumMath.cpp _rgbf
 
@@ -1405,8 +1406,10 @@ Calls @ref Color3::fromLinearRgbInt() on the literal value. Example usage:
 @see @link operator""_rgbaf() @endlink, @link operator""_rgb() @endlink
 @m_keywords{_rgbf rgbf}
 */
-inline Color3<Float> operator "" _rgbf(unsigned long long value) {
-    return Math::unpack<Color3<Float>>(Color3<UnsignedByte>{UnsignedByte(value >> 16), UnsignedByte(value >> 8), UnsignedByte(value)});
+constexpr Color3<Float> operator "" _rgbf(unsigned long long value) {
+    return {((value >> 16) & 0xff)/255.0f,
+            ((value >>  8) & 0xff)/255.0f,
+            ((value >>  0) & 0xff)/255.0f};
 }
 
 /** @relatesalso Magnum::Math::Color3
@@ -1427,7 +1430,8 @@ inline Color3<Float> operator "" _srgbf(unsigned long long value) {
 /** @relatesalso Magnum::Math::Color4
 @brief Float linear RGBA literal
 
-Calls @ref Color4::fromLinearRgbaInt() on the literal value. Example usage:
+Equivalent to calling @ref Color4::fromLinearRgbaInt() on the literal value.
+Example usage:
 
 @snippet MagnumMath.cpp _rgbaf
 
@@ -1439,8 +1443,11 @@ Calls @ref Color4::fromLinearRgbaInt() on the literal value. Example usage:
 @see @link operator""_rgbf() @endlink, @link operator""_rgba() @endlink
 @m_keywords{_rgbaf rgbaf}
 */
-inline Color4<Float> operator "" _rgbaf(unsigned long long value) {
-    return Math::unpack<Color4<Float>>(Color4<UnsignedByte>{UnsignedByte(value >> 24), UnsignedByte(value >> 16), UnsignedByte(value >> 8), UnsignedByte(value)});
+constexpr Color4<Float> operator "" _rgbaf(unsigned long long value) {
+    return {((value >> 24) & 0xff)/255.0f,
+            ((value >> 16) & 0xff)/255.0f,
+            ((value >>  8) & 0xff)/255.0f,
+            ((value >>  0) & 0xff)/255.0f};
 }
 
 /** @relatesalso Magnum::Math::Color4

src/Magnum/Math/Test/ColorTest.cpp

@@ -626,15 +626,26 @@ void ColorTest::data() {
 }
 
 void ColorTest::literals() {
-    constexpr Color3ub a = 0x33b27f_rgb;
-    CORRADE_COMPARE(a, (Color3ub{0x33, 0xb2, 0x7f}));
-
-    constexpr Color4ub b = 0x33b27fcc_rgba;
-    CORRADE_COMPARE(b, (Color4ub{0x33, 0xb2, 0x7f, 0xcc}));
-
-    /* Not constexpr yet */
+    CORRADE_COMPARE(0x33b27f_rgb, (Color3ub{0x33, 0xb2, 0x7f}));
+    CORRADE_COMPARE(0x33b27fcc_rgba, (Color4ub{0x33, 0xb2, 0x7f, 0xcc}));
     CORRADE_COMPARE(0x33b27f_rgbf, (Color3{0.2f, 0.698039f, 0.498039f}));
     CORRADE_COMPARE(0x33b27fcc_rgbaf, (Color4{0.2f, 0.698039f, 0.498039f, 0.8f}));
+
+    /* As the implementation doesn't delegate into unpack() etc in order to be
+       constexpr, test also boundary values to be sure */
+    CORRADE_COMPARE(0xffffff_rgbf, Color3{1.0f});
+    CORRADE_COMPARE(0x000000_rgbf, Color3{0.0f});
+    CORRADE_COMPARE(0xffffffff_rgbaf, Color4{1.0f});
+    CORRADE_COMPARE(0x00000000_rgbaf, (Color4{0.0f, 0.0f}));
+
+    constexpr Color3ub ca = 0x33b27f_rgb;
+    constexpr Color4ub cb = 0x33b27fcc_rgba;
+    constexpr Color3 cc = 0x33b27f_rgbf;
+    constexpr Color4 cd = 0x33b27fcc_rgbaf;
+    CORRADE_COMPARE(ca, (Color3ub{0x33, 0xb2, 0x7f}));
+    CORRADE_COMPARE(cb, (Color4ub{0x33, 0xb2, 0x7f, 0xcc}));
+    CORRADE_COMPARE(cc, (Color3{0.2f, 0.698039f, 0.498039f}));
+    CORRADE_COMPARE(cd, (Color4{0.2f, 0.698039f, 0.498039f, 0.8f}));
 }
 
 void ColorTest::colors() {