Fix the unit tests for Imagick alpha.

These tests were written to use the `assertImageAtAlpha()` helper method, whose
internals were specific to GD. As a result, the tests could not pass. This
changeset introduces GD- and Imagick-specific versions of this method.

Props voldemortensen.
Fixes #24871.

git-svn-id: https://develop.svn.wordpress.org/trunk@30549 602fd350-edb4-49c9-b593-d223f7449a82
Editor CSS: Add missing semicolons.

props cfoellmann.
fixes #30487.

git-svn-id: https://develop.svn.wordpress.org/trunk@30550 602fd350-edb4-49c9-b593-d223f7449a82
Update the Iris color picker to version 1.0.6.

Changes: https://github.com/Automattic/Iris/compare/1.0.4...1.0.6

props mattwiebe.
fixes #30365.

git-svn-id: https://develop.svn.wordpress.org/trunk@30551 602fd350-edb4-49c9-b593-d223f7449a82
Customizer: Don't override `Section.isContextuallyActive()` in `SidebarSection`.

This fixes a bug where empty widget areas get deactivated in the Customizer.

fixes #30378.
see #30235.
props westonruter.

git-svn-id: https://develop.svn.wordpress.org/trunk@30552 602fd350-edb4-49c9-b593-d223f7449a82
Customizer: Only show the default button in the upload control if there is a default attachment.

props celloexpressions.
see #21483.

git-svn-id: https://develop.svn.wordpress.org/trunk@30553 602fd350-edb4-49c9-b593-d223f7449a82
Clear update caches when site language is changed.

The update screen has a re-install button which could show the wrong language locale after the site language was changed. That's because the update data is cached. 
Introduce `_wp_clear_update_cache()` as a helper function to clean up the update cache.

props SergeyBiryukov, ocean90.
fixes #30369.

git-svn-id: https://develop.svn.wordpress.org/trunk@30554 602fd350-edb4-49c9-b593-d223f7449a82