Related to #6177 and #6131. Filed as separate issue because #6177 is getting too big.
Align automatically generated image sizes to browser stats taking into account 2x and 3x displays.
This might seem like a lot, but for most images only the smaller sizes will be generated. For larger images, providing these additional sizes available for the srcset attribute will ensure the browser pulls down the smallest possible image for wide images displayed in wider viewports.
This permanently disassociates the physical image sizes from the classic display widths defined by WordPress. Which makes sense because the old modality of pixel widths defining small/medium/large image sizes no longer applies anyway.
Added duplicate as Core ticket 45406: Add additional default image sizes for srcset to accommodate wider viewports.