Supporting webp format

[znagy]

Allowing webp format for GD

[yugaego]

Thanks for the PR. It's better not to set $quality value here (webp handling should be similar to the one for jpeg, not gif or png).

With the PR-suggested change, end-users won't have control over the resulting webp images quality, even passing $quality to save/render functions. Note that $quality is passed by reference to _save_function. So we get the value of 85 to be "hard-coded", instead of the expected behavior.

[yugaego]

Please check this line: my editor shows trailing spaces after the break;.

[yugaego]

Side note, not related to the PR directly: I really think this error message could be improved. Not handling webp prior this PR, for instance, doesn't mean Installed GD does not support it. Hence the error message is misleading, and could be improved too, whilst we're looking at this part of code.

[yugaego]

Well, I've wrapped up the above said into yugaego@ad4b181 and for now load it in my project with

composer.json

{
    "require": {
        "yurkinx/yii2-image": "dev-webp"
    },
    "repositories": [
        {
            "type": "vcs",
            "url":  "git@github.com:yugaego/yii2-image.git"
        }
    ]
}

The linked branch also adds WebP support to Imagick driver, since compiling imagick with WebP support seems to be a common option nowadays.

For reference, to quick check whether WebP support is enabled on the server, do:

• $ php -r "print_r(gd_info());"
• $ php -r "print_r(Imagick::queryFormats());"