`$file->srcset()` includes upscaled image sizes

Description

If $file->srcset() gets passed one or multiple image width(s) that are larger than the source image, the resulting srcset string includes those sizes as well. The linked thumb is of course not upscaled, but the browser only knows after requesting the image.

Expected behavior

Widths that are larger than the source should be filtered out and the source width as maximum option should be added instead.

To reproduce

With a source image of 750px width

echo $file->srcset([300, 500, 1000, 1500]);

all those sizes are printed. Instead, the result should be 300, 500, 750.

Your setup

Kirby Version

develop branch

Additional context

Things to keep in mind:

If an options array is passed as value, we cannot reliably filter out sizes. So in this case, the behavior should be unchanged. So only the "width as key" and "width as value" cases make sense to handle automatically.
If multiple sizes above the source width are passed, the source width should still only be added once.
If the source width was already passed, it should also not be added a second time.

getkirby / kirby