Clamp the hival parameter of Indexed color space to the range [0; 255]

[calixteman]

Since this value is used to allocate an array, it makes sense to avoid to use too much memory.
From the specs, this value must be in the range [0; 255] (see section 8.6.6.3).
This patch removes the unused property 'highVal'.

[calixteman]

The idea comes from here: https://issues.chromium.org/issues/372749732
/botio test

[moz-tools-bot]

From: Bot.io (Linux m4)

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Received

Command cmd_test from @calixteman received. Current queue size: 0

Live output at: http://54.241.84.105:8877/8a6a2849e38aa69/output.txt

[moz-tools-bot]

From: Bot.io (Windows)

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Received

Command cmd_test from @calixteman received. Current queue size: 0

Live output at: http://54.193.163.58:8877/83ced48cd2873bb/output.txt

[moz-tools-bot]

From: Bot.io (Linux m4)

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Failed

Full output at http://54.241.84.105:8877/8a6a2849e38aa69/output.txt

Total script time: 30.90 mins

• Unit tests: Passed
• Integration Tests: Passed
• Regression tests: FAILED

  different ref/snapshot: 14
  different first/second rendering: 1

Image differences available at: http://54.241.84.105:8877/8a6a2849e38aa69/reftest-analyzer.html#web=eq.log

[moz-tools-bot]

From: Bot.io (Windows)

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Failed

Full output at http://54.193.163.58:8877/83ced48cd2873bb/output.txt

Total script time: 46.36 mins

• Unit tests: Passed
• Integration Tests: FAILED
• Regression tests: FAILED

  different ref/snapshot: 5

Image differences available at: http://54.193.163.58:8877/83ced48cd2873bb/reftest-analyzer.html#web=eq.log

[Snuffleupagus]

Spelling nit: Change then -> the in the commit message.

r=me, thank you.