[webview_flutter] WIP Webview transparent background

[renefloor]

Adds the option to set the background of a webview to transparent.

Fixes flutter/flutter#29300
Updated PR #2895 with latest master.

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[stuartmorgan]

See my comments in #3431; this is not the way we should add this given the existing request for other background colors.

That PR is currently active, so I suggest just closing this and adding review comments to the other PR as necessary.