Ensure correct page size

I have found out in the most painful way possible that a lot of Linux desktop components (and I mean practically all of them) tend to inexplicably default to the US Legal paper size, especially on en_US locale, insidiously resizing the picture while I’m trying to print it. US Legal is close enough to A4 that you wouldn’t notice the error immediately.

For example, pictured on the left is the 32mm wide calibration mask, printed from an A4 SVG via Chromium with paper size set to A4 in the printing dialog. On the right is the same mask, printed via Inkscape:

This also gives rise to all kinds of fascinating artifacts, such as 0.15mm line being thinner than 0.1mm one:

Watch out!