Optimise landscape plotting

[hugovk]

Testing on ~600 activity files, already cached.

Before: 65.50s

$ time stravavis "activities/2025*" --activities_path activities.csv --bbox helsinki.bbox -o strava2025 --plot landscape
Processing data...
Cache filename: /var/folders/c8/hl_96fcs0hjgr29njx33x20c0000gn/T/stravavis/cached_activities_1e275bfa9783382db17fdaf6a767ebc8.pkl
Loaded cached activities
Processing activities...
Plotting landscape...
Processing tracks ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100% 0:00:20
Plotting activities ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100% 0:00:42
Saved to strava2025-landscape.png
stravavis "activities/2025*" --activities_path activities.csv --bbox  -o     65.50s user 0.72s system 99% cpu 1:06.34 total

After: 24.19s

$ time stravavis "activities/2025*" --activities_path activities.csv --bbox helsinki.bbox -o strava2025 --plot landscape
Processing data...
Cache filename: /var/folders/c8/hl_96fcs0hjgr29njx33x20c0000gn/T/stravavis/cached_activities_1e275bfa9783382db17fdaf6a767ebc8.pkl
Loaded cached activities
Processing activities...
Plotting landscape...
Plotting activities ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100% 0:00:21
Saved to strava2025-landscape.png
stravavis "activities/2025*" --activities_path activities.csv --bbox  -o     24.19s user 0.44s system 98% cpu 24.883 total