LibPDF: Convert LAB values to bytes differently

Gfx::ICC::Profile's current API takes bytes, so we need to do some
contortions for LAB values to go through.

This will probably become nicer once we implement all the backward
transforms in Gfx::ICC::Profile, but for now let's hack it in
on the LibPDF side.

Makes colors in 0000651.pdf looks good, especially on pages 1 and 7-12.

Userland/Libraries/LibPDF/ColorSpace.cpp

@@ -499,9 +499,21 @@ PDFErrorOr<Color> ICCBasedColorSpace::color(ReadonlySpan<Value> arguments) const
         s_srgb_profile = TRY(Gfx::ICC::sRGB());
 
     Vector<u8> bytes;
-    for (auto const& arg : arguments) {
+    for (size_t i = 0; i < arguments.size(); ++i) {
+        auto const& arg = arguments[i];
         VERIFY(arg.has_number());
-        bytes.append(static_cast<u8>(arg.to_float() * 255.0f));
+        float number = arg.to_float();
+
+        if (m_profile->data_color_space() == Gfx::ICC::ColorSpace::CIELAB) {
+            // CIELAB channels go from 0..100 and -128..127 instead of from 0..1.
+            // FIXME: We should probably have an API on Gfx::ICC::Profile that takes floats instead of bytes and that does this internally instead.
+            if (i == 0)
+                number /= 100.0f;
+            else
+                number = (number + 128.0f) / 255.0f;
+        }
+
+        bytes.append(static_cast<u8>(number * 255.0f));
     }
 
     auto pcs = TRY(m_profile->to_pcs(bytes));