CI: Use Meson to build librsvg for Visual Studio builds

Hi,

Note that this MR builds on top of !949 (merged), as I believe things there should go before the changes in this MR.

This attempts to update the vs2017-x64 CI to use Meson instead of NMake, so that we verify that changes continue to build on Visual Studio builds especially after we will make the full switch to Meson.

This also updates meson/query-rustc-native-libs.py to work on pre-Python-3.9.x (i.e. Meson is invoked with Python-3.8.x or earlier), as the vs2017-x64 CI is currently using Python-3.8.x, and fixes the install script for the SVG GdkPixbuf loader on Visual Studio-style builds.

With blessings, thank you!