Compute "sub" positions/dimensions correctly

In my branch https://gitlab.gnome.org/federico/librsvg/tree/wilber-dimensions I did this:

• Enabled the /dimensions/sub/rect with transform test again.
• Disabled the code in draw.rs that reduces Cairo's tolerance. This makes that test pass again (... for the first time in 8 years).
• Added @Jehan's test for fetching the dimensions of a Wilber icon.

This last test fails; we are not computing the icon's position/dimensions correctly. I think this is because the icon is a <g> with a transform, but I'm not 100% sure yet.

I'd like to make this a meta-bug for #128 (closed) and #134 (closed) as well.