Electron capturePage needs a delay or capturePage Images may not have all content

[codecounselor]

The problem is that when capturePage is used to export a PNG image, the contents of the page have been found to be partially missing in some cases.

Someone made an attempt to add a view-painted event to webContents in place of a timeout but it proved not to work and it was removed.
The history is captured here: electron/electron#6622

This task is to add a small 100ms timeout before invoking capturePage when a PNG is being exported.

If a client is using the waitForJSEvent option they can also insert a timeout before emitting the ready event.

Allow the delay to be overridden with a ELECTRONPDF_PNG_CAPTURE_DELAY env property