FX.140 Resin slicing: faster on big screens and dense miniature meshes, and less memory

A tester reported the resin slicer crashed while slicing; they later said the model was a dense mesh from a miniatures set, a few MB, not regular geometry. The cause is not confirmed and no crash was reproduced, but tests found three real problems. (1) Drawing: the page spent most of its time turning each layer into a 12K picture (59 million pixels a layer); a 100 mm part at 0.05 mm took over 280 s. Now only the rows a layer lights are touched, one buffer is reused, and dark rows are written as single runs: a 60 mm part 151 s to 27 s, the 100 mm one over 280 s to 100 s. (2) Dense meshes: the island check ran an exact overlap test for every small piece of every layer (40 of 52 s on a 40,000-triangle noisy ball with supports); the overlap of the whole layers is now found once and cut into pieces, same island counts as before (251 = 251 on a test mesh), 52 s to 22 s. The cavity of a hollow part is worked out on an outline simplified by half a pixel (about 25 percent faster, wall within half a pixel). (3) Memory: a 40-piece, 139,000-triangle mesh at 0.03 mm, hollow with supports, peaked at 1.98 GB (enough to kill a browser tab), because every layer's pieces were kept to the end; temporary shapes are now given back at once: 1.20 GB, and it finishes in about 4.5 minutes where it ran past 7. Layers are identical to before the change except the half-pixel cavity. With RSN.8 the slicing also runs on a worker thread, so the page cannot freeze. Not tried on a real printer or on the tester's own file.