make: Adopted consistent codemap dimensions

Tweaked: 1400x750 -> 1125x525 (1.5x codemapsvg.py's default)

This is now derived (1.5x) from the default dimensions in codemapsvg.py.
This matches the dimensions that ended up used for the preliminary v3
benchmarks, which are a bit more convenient on devices with smaller
screens.

As for where the 750x350 resolution came from, I'm not entirely sure.
Maybe a random Matplotlib example? It approximates a 2:1 aspect ratio
but with 25 pixels carved out for margins.

Note we like wide aspect ratios over pretty aspect ratios like 16:9,
golden ratio, etc, here:

1. We often cram things into the margins (legends, stack usage, etc)

2. English text is much wider than it is tall (this commit message has
   an aspect ration of ~3:1), so wider aspect ratios help readability

Makefile

@@ -462,7 +462,7 @@ summary-diff sizes-diff: $(OBJ) $(CI)
 
 ## Generate a codemap svg
 .PHONY: codemap
-codemap: CODEMAPFLAGS+=-W1400 -H750 --dark
+codemap: CODEMAPFLAGS+=-W1125 -H525 --dark
 codemap: $(BUILDDIR)/lfs3.codemap.svg
 
 ## Generate a tiny codemap, where 1 pixel ~= 1 byte