sl0ppty: walk the row, not screen_at()

shade_apply() reached every cell through screen_at(x, y), which re-derives
the row base and bounds-checks a coordinate the loop already guarantees --
once per cell, per shader, per frame.

Clipping the rect to the screen once, up front, makes the pointer arithmetic
provably in range, so the inner loop can walk a row. Same output, and the
clip is not a behaviour change: the layout never asks for a rect that needs
it, and a caller that did was getting NULL-skipped cells before.

Measured with .scratch/shprof.c on a 200x50 pane and a three-pass chain:
8.0 -> 6.1 ns per cell-pass, 0.24 -> 0.18 ms per frame.

That profile also says where the time is *not*: `vignette`, which computes
squared distances and a divide, is cheaper than `dim`, which computes
nothing. The arithmetic in these shaders is free; what costs is per-cell
overhead and the colour mixing. Anyone arriving here to precompute a
per-cell amount table should read the numbers first -- it buys nothing.

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