Cover Image: Add control for overlay opacity

[melchoyce]

What if instead of an on/off toggle, we added a range slider for opacity?

Could snap it to percentages of 10, maybe, and then allow any number in the input field.

[westonruter]

Also, this cover image control would particularly benefit from having the color options added in #2036, as it could be that if you have selected a light image then all you need is to select a dark color for the text. In that case, you wouldn't need any dimming at all.

[melchoyce]

Probably overkill, but could also be fun to add some CSS image filter options, like grayscale, sepia, and blur.

[WPDevHQ]

Could also do with further controls on fixed image like "image position" i.e contain, cover e.t.c

[karmatosed]

Closing this as it's merged in :)