LibGfx/ICC: Add a built-in LAB mft2 profile

This is even weirder than the XYZ mft2 profile added in an
earlier commit in this PR:

mft2 uses a "legacy 16-bit PCSLAB encoding" instead of the "normal"
16-bit ICC PCSLAB encoding, and this being an identity mapping leaks
this into the outputs. We again linearly map this internal 16-bit
encoding to 8 bits.

This legacy encoding maps 0 to 0.0 and FF00 to 1.0 for L
(and FFFF to a bit more than 1.0), and 0 to -128.0 and FF00 to
127.0 for a* and b* (and FFFF to a bit more than 127.0).

This means this produces different values than our built-in LAB mft1
profile:

    % echo 'pcslab(100, 127, -128)' | \
          icc -n LAB_mft2 --stdin-u8-from-pcs | \
          icc -n LAB --stdin-u8-to-pcs
    pcslab(99.60784, 126, -128)

Staying in LAB for both steps is exact, of course:

    % echo 'pcslab(100, 127, -128)' | \
          icc -n LAB --stdin-u8-from-pcs | \
          icc -n LAB --stdin-u8-to-pcs
    pcslab(100, 127, -128)

Staying in LAB_mft2 for both steps is also exact, up to u8
resolution (100 and 127 don't exactly map to an u8 value with
our "16-bit legacy PCSLAB encoding mapped to u8" encoding):

    % echo 'pcslab(100, 127, -128)' | \
          icc -n LAB_mft2 --stdin-u8-from-pcs | \
          icc -n LAB_mft2 --stdin-u8-to-pcs
    pcslab(99.99693, 126.99219, -128)

This is just because the same PCSLAB value is encoded slightly
differently in the mft1 and mft2 LAB encodings:

    % echo 'pcslab(100, 127, -128)' | \
          icc -n LAB --stdin-u8-from-pcs
    255, 255, 0

    % echo 'pcslab(100, 127, -128)' | \
          icc -n LAB_mft2 --stdin-u8-from-pcs
    254, 254, 0

(There's no XYZ mft1 encoding, so at least we don't have mft1 and mft2
profiles that have different output.)

Use this profile to add a roundtrip test for Lut16TagData when PCS
is PCSLAB.

Author: nico

Tests/LibGfx/TestICCProfile.cpp

@@ -300,6 +300,12 @@ TEST_CASE(roundtrip_lab_mft1)
     test_roundtrip(*profile);
 }
 
+TEST_CASE(roundtrip_lab_mft2)
+{
+    auto profile = TRY_OR_FAIL(Gfx::ICC::IdentityLAB_mft2());
+    test_roundtrip(*profile);
+}
+
 TEST_CASE(roundtrip_xyz_mft2)
 {
     auto profile = TRY_OR_FAIL(Gfx::ICC::IdentityXYZ_D50());

Userland/Libraries/LibGfx/ICC/WellKnownProfiles.cpp

@@ -112,6 +112,44 @@ ErrorOr<NonnullRefPtr<Profile>> IdentityLAB()
     return Profile::create(header, move(tag_table));
 }
 
+ErrorOr<NonnullRefPtr<Profile>> IdentityLAB_mft2()
+{
+    // Identity mapping between CIELAB and PCSXYZ, using an unnecessarily large mft2.
+
+    auto header = profile_header(ColorSpace::CIELAB, ColorSpace::PCSLAB);
+    header.device_class = DeviceClass::ColorSpace;
+
+    OrderedHashMap<TagSignature, NonnullRefPtr<TagData>> tag_table;
+
+    TRY(tag_table.try_set(profileDescriptionTag, TRY(en_US("SerenityOS Generic LAB, mft2"sv))));
+    TRY(tag_table.try_set(copyrightTag, TRY(en_US("Public Domain"sv))));
+
+    // mft1 is plenty; this is just for testing mft2 LAB codepaths.
+    EMatrix3x3 e_matrix = identity_matrix();
+
+    Vector<u16> input_tables;
+    for (int c = 0; c < 3; ++c) {
+        // mft2 allows between 2 and 4096 entries. If there are just two values, it linearly maps between them.
+        input_tables.append(0);
+        input_tables.append(65535);
+    }
+
+    auto clut_values = make_2x2x2_cube<u16>();
+    Vector<u16> output_tables = input_tables;
+
+    auto mft2 = TRY(try_make_ref_counted<Lut16TagData>(0, 0, e_matrix,
+        3, 3, 2,
+        2, 2,
+        move(input_tables), move(clut_values), move(output_tables)));
+    TRY(tag_table.try_set(AToB0Tag, mft2));
+    TRY(tag_table.try_set(BToA0Tag, mft2));
+
+    // White point.
+    TRY(tag_table.try_set(mediaWhitePointTag, TRY(XYZ_data(header.pcs_illuminant))));
+
+    return Profile::create(header, move(tag_table));
+}
+
 ErrorOr<NonnullRefPtr<Profile>> IdentityXYZ_D50()
 {
     // Identity mapping between nCIEXYZ and PCSXYZ.

Userland/Libraries/LibGfx/ICC/WellKnownProfiles.h

@@ -15,6 +15,7 @@ class Profile;
 class ParametricCurveTagData;
 
 ErrorOr<NonnullRefPtr<Profile>> IdentityLAB();
+ErrorOr<NonnullRefPtr<Profile>> IdentityLAB_mft2();
 ErrorOr<NonnullRefPtr<Profile>> IdentityXYZ_D50();
 
 ErrorOr<NonnullRefPtr<Profile>> sRGB();

Userland/Utilities/icc.cpp

@@ -381,6 +381,8 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
         if (!name.is_empty()) {
             if (name == "LAB")
                 return Gfx::ICC::IdentityLAB();
+            if (name == "LAB_mft2")
+                return Gfx::ICC::IdentityLAB_mft2();
             if (name == "sRGB")
                 return Gfx::ICC::sRGB();
             if (name == "XYZ")