LibPDF: Make indexed lab color spaces work

We have to use the scale of the underlying color space. For most
color spaces, that's [0.0, 1.0], but not for lab color spaces.

The spec comment at the start of this function already tells us
what to do, but the code used to not do it. Now it does.

Author: nico

Userland/Libraries/LibPDF/ColorSpace.cpp

@@ -703,7 +703,8 @@ PDFErrorOr<NonnullRefPtr<ColorSpace>> IndexedColorSpace::create(Document* docume
         return Error { Error::Type::MalformedPDF, "Indexed color space expects stream or string for third arg" };
     }
 
-    size_t needed_size = (hival + 1) * base->number_of_components();
+    size_t const n = base->number_of_components();
+    size_t needed_size = (hival + 1) * n;
     if (lookup.size() - 1 == needed_size) {
         // FIXME: Could do this if lookup.size() > needed_size generally, but so far I've only seen files that had one byte too much.
         lookup.resize(needed_size);
@@ -714,9 +715,10 @@ PDFErrorOr<NonnullRefPtr<ColorSpace>> IndexedColorSpace::create(Document* docume
     }
 
     Vector<float> lookup_float;
+    auto decode = base->default_decode();
     lookup_float.resize(lookup.size());
     for (size_t i = 0; i < lookup.size(); ++i)
-        lookup_float[i] = lookup[i] / 255.0f;
+        lookup_float[i] = mix(decode[2 * (i % n)], decode[2 * (i % n) + 1], lookup[i] / 255.0f);
 
     auto color_space = adopt_ref(*new IndexedColorSpace(move(base)));
     color_space->m_hival = hival;