Add a tolerance to width and height in ImageDimensionsTest::testImageDimensions()

Problem/Motivation

This was added in #2918570: Drupal\KernelTests\Core\Image\ToolkitGdTest fails on PHP 7.1.x-dev and 7.0.x-dev following a PHP bugfix and never resolved. An attempt in #2921123: Adjust Rectangle class to calculate rotated image dimensions according to libgd 2.2.2+ stalled.

    $effect_id = $style->addImageEffect($effect);
    $style->save();
    // @todo Uncomment this once
    //   https://www.drupal.org/project/drupal/issues/2670966 is resolved.
    // $this->assertEqual($this->getImageTag($variables), '<img src="' . $url . '" width="41" height="41" alt="" class="image-style-test" />');
    $this->assertFalse(file_exists($generated_uri), 'Generated file does not exist.');
    $this->drupalGet($this->getAbsoluteUrl($url));
    $this->assertResponse(200, 'Image was generated at the URL.');
    $this->assertTrue(file_exists($generated_uri), 'Generated file does exist after we accessed it.');
    $image_file = $image_factory->get($generated_uri);
    // @todo Uncomment this once
    //   https://www.drupal.org/project/drupal/issues/2670966 is resolved.
    // $this->assertEqual($image_file->getWidth(), 41);
    // $this->assertEqual($image_file->getHeight(), 41);

Steps to reproduce

Proposed resolution

Different versions of the GD library bundled or linked in PHP provide slightly different results for the image rotation function, so we can not do an exact match check here. We tolerate a max difference of two pixels.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet