Bilinear/trilinear mipmapping down-sampling are working as bilinear

[dacap]

In Aseprite v1.2.34 when we introduced down-sampling with mipmapping it worked correctly:

Screencast.2024-02-19.16.53.19.mp4

This doesn't work anymore in v1.3 (probably when we updated Skia from m96 to m102).

Related: https://igarastudio.zendesk.com/agent/tickets/5813