Add webp support to Image Manipulation Class

[nicojmb]

Each pull request should address a single issue and have a meaningful title.

Description
Support webp image in Image Manipulation Class

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• User guide updated
• [x ] Conforms to style guide

---------Remove from here down in your description----------

Notes

• Pull requests must be in English
• If the PR solves an issue, reference it with a suitable verb and the issue number
  (e.g. fixes 12345)
• Unsolicited pull requests will be considered, but there is no guarantee of acceptance
• Pull requests should be from a feature branch in the contributor's fork of the repository
  to the develop branch of the project repository

[lonnieezell]

It is possible for Imagemagick to support webp if it's compiled in so please make sure the Imagemagick handler also works with it or throws the error if not.

[michalsn]

@nicojmb we have two things to fix here:

• The first - tests. If you look at Travis, we have 3 tests that are failing now, we have to change constant from IMAGETYPE_PNG to IMG_PNG since this is the correct one returned by getimagesize() function.
• The second thing that I didn't notice earlier is that your commits are not signed and we can't merge your code. Please check this page to learn more about code signing: https://github.com/codeigniter4/CodeIgniter4/blob/develop/contributing/signing.rst

@MGatner is there a way to sign the code right now, to make it mergeable? I think you showed me once some instructions but I didn't succeed make them work for me (it was probably my fault). So, I'm just wondering is there a way to "fix" this PR in this state.

[MGatner]

@michalsn you can add credentials to GitHub post-push and it will verify previous commits, but if the commits weren’t signed already that won’t work. Anyone could pull the fork, git reset develop, then re-add and re-commit a signed version and make a new PR. Or Lonnie could just override the verification requirement 😇

[MichalPB1]

Can it be merged? This is something that is useful to me :)

[michalsn]

Right now we can't merge it since it's not signed and tests are failing. If nothing will change, I will probably wait till the weekend and try to fork this branch and re-commit.

[lonnieezell]

Yeah, the biggest problem is the tests. I am happy to override. And am thinking about removing that requirement, anyway, actually.

[michalsn]

Yes, removing the code signing requirement would probably be a good idea.