Add flag to disable overlays

[harryterkelsen]

Adds an option to disable overlay canvases.

We create an overlay canvas (and new GL context) for each platform view, but we are hitting issues since the GL resources aren't shared between contexts.

This adds a flag to disable overlay canvases to work around the issue, but it may cause rendering glitches in the case where content is supposed to be overlaid on the platform view.

Workaround for flutter/flutter#86809

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[yjbanov]

As discussed offline, instead of forcing maximumOverlaySurfaces to be >= 2, let's treat maximumOverlaySurfaces == 1 as "overlays not supported". This will obviate the need for a second environment variable.

[harryterkelsen]

Done. PTAL