font/sprite: replace pixman with z2d, extend Box coverage

More complete coverage of the Symbols For Legacy Computing block,
including characters from Unicode 16.0.

Pixman and the web canvas impl for Canvas have been removed in favor of
z2d for drawing, since it has a nicer API with more powerful methods,
and is in Zig with no specific platform optimizations so should compile
to wasm no problem.

nix/devShell.nix

@@ -41,7 +41,6 @@
   libXi,
   libXinerama,
   libXrandr,
-  pixman,
   zlib,
   alejandra,
   pandoc,
@@ -61,7 +60,6 @@
       harfbuzz
       libpng
       oniguruma
-      pixman
       zlib
 
       libX11
@@ -126,7 +124,6 @@ in
         harfbuzz
         libpng
         oniguruma
-        pixman
         zlib
 
         libX11