on-disk raster graphics should all load and render in a HiDPI-friendly way

[jwise]

todo on this front:

• all icons, when looking for a scale to load textures, should ask for the point size they want, and have the texture loader come up with a path that satisfies the current HiDPI situation and satisfies the requested point size (i.e., the toolbar icon loader should not construct the path itself)
• ... either that, or all icons should just be rendered in 2x, and the path names should be in point size, and then on 1x systems, we can use some sort of system-integrated mechanism to scale icons down (perhaps some kind of "graphics processing unit"? do computers have those these days?)
• there should at least be a shell script to rerender all of the .svgs into their target PNGs, and really, that should probably be a build step, rather than having PNG binary artifacts checked in that are derived from SVGs