For some reasone when grandparent of ImageBase__img has
border-radius and overflow: hidden we see background
of ImageBase at the corner behind ImageBase__img, especially
it is visible when the ImageBase__img has same colors as page
background.

Seems like this is due to ImageBase__image is absolutely positioned,
so, parent background is partly visible at corners.

The solution is to make ImageBase__img static.
This is actully how it used to be in Avatar component
before it has been refactored to ImageBase.
see ae00ed7 and cdc74df
where .Avatar__img has been relatively positioned.

Also, we remove negative z-index from img as it doesn't change much
results in e2e tests. Negative z-index was introduced in 9d47c34

To avoid antializing issue with background
we show background on theparent element of img (.ImageBase)
while img is loading or in case there is no src at all.
But when img is loaded we disable background on .ImageBase
and show background on img itself