Downscale textures correctly when using graphics-quality=medium

[dupraz]

Since version 2.0, Factorio no longer ships with half-resolution rendered textures (zoom=1), and either downscales them using the GPU at runtime (graphics-quality=high), or using the CPU at startup (graphics-quality=medium) when building the half-resolution atlas to reduce VRAM usage.

When downscaling is done using the CPU at startup, no filtering is applied (nearest-neighbor), which results in artifacts, severe aliasing, and poor spatial stability, which were not present in the reference half-resolution render from 1.1.
When downscaling the high-resolutions textures correctly, either using the GPU filtering which is used at runtime, or with correct settings (say using Lanczos sampling), these artifacts are no longer present, and is practially indiscernible from the reference 1.1 render (some degree of aliasing is still present even when using the GPU at graphics-quality=high).

Currently, playing the game with graphics-quality=medium due to low VRAM results in a very noticeable, visually degraded experience which could easily be fixed at no further VRAM cost.

You can see the comparison for yourself below.

Runtime (GPU) downscaling:

factorio_2_zoom_1_quality_2.png (2.54 MiB) Viewed 140 times

Startup (CPU) downscaling:

factorio_2_zoom_1_quality_1.png (2.48 MiB) Viewed 140 times

Comparison (GIF, dithered):

comparison.gif (2.24 MiB) Viewed 140 times