fix: a density grid is bounded in cells, not by the window's pixels

A grid was the axes rect in device pixels, so dragging a window wide made the
image wider with it -- and the per-frame pass over it is O(cells): the fade
multiplies every cell, a host's ink()/peak() read every cell, and a frame the
fade moved converts and uploads every cell as one byte. At 2x device scale a
1520x850 window's axes rect is 2877 device pixels wide, so four rows of one
trace are 4.5 M cells and 4.9 MB uploaded a frame; measured headless on
llvmpipe with a 2 s window, 12.9 ms of tick and 12.8 ms of draw a frame, every
frame under a time fade -- a cost that follows the window and has no ceiling.

The grid is now the axes rect up to kDensityCellCap (2^18 cells, a mebibyte of
counts), scaled down uniformly past it so cells stay square and the picture
keeps the axes' aspect, and magnified back over the rect with a linear filter:
a cell is a count, so a fragment between two of them is between their two ink
levels, which a point sample cannot give once a cell is several pixels wide.
The same four rows at the cap are 1.05 M cells, 3.9 ms of tick and 5.6 ms of
draw and 1.0 MB uploaded, while a grid under the cap is untouched -- a 640x360
viewport's 573x313 axes is still 179 349 cells, one a device pixel.

The image's footprint is the axes rect rounded up to whole device pixels on
both sides of the pipe, so a grid under the cap keeps the exact
one-texel-a-pixel mapping it has always had (sx/sy come out exactly 1) and only
a capped grid reads its cell index through a scale. Moving the decay itself to
the GPU was considered and left out: the CPU grid is what ink()/peak() read,
what the deposit adds to and what the per-sweep dedupe keys on, so a GPU image
would have to be reconciled with it per frame -- a larger change than this
bound, and not one to half-do.

Tests: a_density_grid_is_capped_whatever_the_window reads the bound off a
3840x2160 viewport (261 504 cells of a 3784x2138 axes, aspect kept),
a_capped_density_is_magnified_over_the_axes_rect reads the screen side of it
(569x460 cells covering all 832 of the rect's columns), and the density cases
now assert their capped figures. 399 cases, 0 failed.

include/plot/render/shaders.hpp

@@ -76,9 +76,9 @@ extern const char* const kTextFS;
 /// nothing to upload (the density's own bytes go to a texture, not to a buffer).
 ///
 /// Uniforms: `u_viewport` (vec2, pixels), `u_origin` (vec2, the quad's top-left corner in `Rect`
-/// pixels), `u_size` (vec2, its size in pixels -- the grid's own columns and rows, so one texel
-/// is one pixel on screen and no filtering is needed), `u_color` (vec4), `u_density` (sampler2D,
-/// unit 0).
+/// pixels), `u_size` (vec2, its size in pixels -- the axes rect the image is drawn over, which the
+/// texture's cells are spread across: one texel a cell while the rect is under `kDensityCellCap`,
+/// and a filtered magnification past it), `u_color` (vec4), `u_density` (sampler2D, unit 0).
 ///
 /// The fragment shader multiplies the sampled single-channel image into `u_color`'s alpha, which
 /// is the whole of the brightness mapping: a cell's count was turned into a luminance byte on the