Use golang.org/x/image/vector directly

[redawl]

When rasterx is used with Filler, it uses golang.org/x/image/vector rasterizer. We can improve performance by using the rasterizer directly. This saves on the conversions from Int26_6 to float etc.

Unfortunately, it doesn't draw pixel perfect to the old snapshot tests. IMO it is probably more accurate, since a lot of float operations are now gone (I can't tell the difference visually).

Benchmark:

redawl@workstation ~/projects/render $ benchstat /tmp/{old,new}.txt
goos: linux
goarch: amd64
pkg: github.com/go-text/render
cpu: AMD Ryzen 7 7800X3D 8-Core Processor
                           │ /tmp/old.txt │            /tmp/new.txt             │
                           │    sec/op    │   sec/op     vs base                │
DrawString/Lorem_Ipsum-16     2.275m ± 0%   1.388m ± 0%  -38.98% (p=0.000 n=10)
DrawString/Arabic_Ipsum-16    4.262m ± 1%   2.501m ± 0%  -41.32% (p=0.000 n=10)
DrawString/Hello_World-16     272.1µ ± 1%   206.3µ ± 1%  -24.18% (p=0.000 n=10)
geomean                       1.382m        894.8µ       -35.25%

[benoitkugler]

Looks very good to me, and the new tests snapshots are OK.

[andydotxyz]

I can barely tell the difference but could be persuaded that the latter is a tad sharper.
Either way the performance improvement is good news.

It will break all our golden masters in Fyne too - but these things do happen some times, we can group a few together for the next big release.