render_node_thumbnail_with draws into a context of size.clamp(1,
MAX_CONTEXT_SIDE) pixels but reported width and height as the requested
size, so a size of 0 gave a 0x0 image holding 4 bytes. The view was
also built from the requested size, and the fit scale (size - 2) /
extent was 0 or negative at sizes 1 and 2, so the art drew nothing or
mirrored.
The thumbnail now reports the side it draws, builds the view from it,
and drops the 1-pixel margin when the side is too small to keep one.
Test: node_thumbnail_reports_the_size_it_draws.
dotCooCoo
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