Allow selecting the original image when creating a responsive image style

There is a fallback image description added in #2334387: UI changes to support current responsive image standards.

The none (original image) change is a separate improvement, and that can continue on here.

Needs work and needs a test to make sure original image works as expected

Patch rerolled, original added to tests

Description is already added, removed it from the patch

Is breaking on empty string, will need to use a constant

The option for the original image can't be an empty string because of the code in ResponsiveImageStyle::isEmptyImageStyleMapping. New patch attached.

Patch with fixed unit test. This patch will still fail because I discovered an other issue while working on this: #2573221: Keyed mapping static cache is not rebuilt when overwriting an existing mapping

Also, I just noticed we're unit testing this, but not testing the actual output with a mapping using the original image, so more tests are needed.

New patch with tests. Will still fail because of #2573221: Keyed mapping static cache is not rebuilt when overwriting an existing mapping.

Small nit picks

1. +++ b/core/modules/responsive_image/responsive_image.module
   @@ -456,7 +457,12 @@ function responsive_image_get_mime_type($image_style_name, $extension) {
   +  if ($image_style_name == RESPONSIVE_IMAGE_ORIGINAL_IMAGE) {
   

   ===

2. +++ b/core/modules/responsive_image/src/ResponsiveImageStyleForm.php
   @@ -84,6 +84,7 @@ public function form(array $form, FormStateInterface $form_state) {
   +    $image_styles[RESPONSIVE_IMAGE_ORIGINAL_IMAGE]=  t('- None (original image) -');
   

   $this->t

3. +++ b/core/modules/responsive_image/src/Tests/ResponsiveImageAdminUITest.php
   @@ -77,7 +77,13 @@ public function testResponsiveImageAdmin() {
   +      [
   +        RESPONSIVE_IMAGE_EMPTY_IMAGE,
   +        RESPONSIVE_IMAGE_ORIGINAL_IMAGE,
   +      ],
   

   1 line is ok

4. +++ b/core/modules/responsive_image/src/Tests/ResponsiveImageAdminUITest.php
   @@ -85,8 +91,17 @@ public function testResponsiveImageAdmin() {
   +          '//select[@name=:name]//option[@value=:style]',
   +          [
   

   move [ up

5. +++ b/core/modules/responsive_image/src/Tests/ResponsiveImageAdminUITest.php
   @@ -85,8 +91,17 @@ public function testResponsiveImageAdmin() {
   +          ]
   +        ));
   

   move ] down

New patch addressing #29.

4 & 5: Didn't do that because indentation would be wrong then.

I'm when when the bot is happy

Committed and pushed to 8.0.x. Thanks!

Only thing I wonder is about swapping the ordering so that original image is before empty image, but that could always be a follow-up and/or an improvement in 8.1.

Follow-up created at #2580113: Change order of original and empty image