LibGfx/ILBM: Ensure CMAP chunk size matches expected value

The color map should be 3 bytes per pixel and should contain
`2^nPlanes` pixels. We now return an error if the color map isn't the
size we expect.

Tests/LibGfx/TestImageDecoder.cpp

@@ -154,6 +154,7 @@ TEST_CASE(test_ilbm_malformed_header)
 TEST_CASE(test_ilbm_malformed_frame)
 {
     Array test_inputs = {
+        TEST_INPUT("ilbm/incorrect-cmap-size.iff"sv),
         TEST_INPUT("ilbm/incorrect-uncompressed-size.iff"sv),
         TEST_INPUT("ilbm/missing-body-chunk.iff"sv)
     };