Exports the full Equal Earth map as a ~4K PNG via an off-screen RenderTexture2D,
reusing the existing Map2D/Overlays drawing code (already parameterized by an
arbitrary target Rectangle) at scale. Extracted the overlay-drawing body of
renderMap2D() into a shared, scale-parameterized Viewer::drawMapOverlays() so
on-screen and export rendering can never diverge.
Fixed a real bug found while testing: the toolbar button called exportMapImage()
synchronously from inside drawToolbar()'s active scissor mode (for its scroll
panel), clipping the export to a tiny corner. Now deferred via a pending flag
and run right after drawToolbar() returns.
Jonas Reith
ddeba0dea8