Optimize the editor icon generation

Created by: Calinou

Follow-up to #35325.

Icons are no longer upsampled when using an integer editor scale. This makes some icons slightly less crisp, but the icons themselves can be adjusted to mitigate this. When using a non-integer editor scale setting, upsampling is kept as it improves crispness in a far more visible manner.

When upsampling is disabled, this speeds up the theme generation by about 100 ms on average, making the project manager and editor start slightly faster. This also speeds up switching between themes.