Implement dpi support in litehtml

[matzipan]

Currently pt to px conversion is done 1 to 1 in the litehtml_rust layer.

The correct approach is something like this:

• https://chromium.googlesource.com/chromium/src/+/d7f901be67d87ffc89d978bca565a3f42d3a8388
• https://chromium.googlesource.com/chromium/src/+/06006236fb05ca3ba187da6a09fc7696f27f5476/ui/gtk/gtk_ui.cc
• https://gtk-rs.org/gtk4-rs/git/docs/gtk4/struct.Settings.html#gtk-xft-dpi