#2452 Winter textures: a couple minor bugs

I noticed that a couple winter textures stand out of the rest too much:

1. Conifers seem to reuse summer texture, when imho they should get a bit "duller" in winter.
2. Grass runways seem to keep their summer texture, which is dirt for low shader level and grass for high shader level.

Example: --airport=EDHE --season=winter

[Fernando García Liñán]

ALS keeps track of the snow level separately through the "Climate" dialog, so I think the separate winter textures are largely unmaintained.