test(demos): render goldens at 1920x1080

test(demos): render goldens at 1920x1080

Raise the DemoGoldens render resolution from 640x480 to 1920x1080
(full HD) so regressions are caught at the resolution the demos
actually run at, and regenerate every golden image at the new
size. Tolerances are unchanged (per-channel 16, 0.005 differing-
pixel fraction; 32 for SDF text scenes).