RTL: fix image resizing handles in RTL mode

[yoavf]

Description

Prevent the handle positions in the CSS from being flipped in RTL mode.
This is necessary after #3844 since the position of the handles is handled in JS.

Screenshots (jpeg or gifs if applicable):

Before:

After:

Types of changes

CSS change to prevent automatic flipping of CSS properties.
The ! before the rtlcss directive is necessary since the positions are set in the css with !important. Actually, because of scss.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows has proper inline documentation.

[mtias]

Looks good to me