Optimize shadow rendering: static object draw state caching and low-res far shadows option

[adfriz]

This should improve shadow rendering and lower RAM usage.

1. Caching:
   • Cache active object, material, and texture bindings in Shadows.cs
   • Skip redundant GL binds and uniform updates to reduce CPU/driver overhead.
2. Low-res Far Shadows Option:
   • Add "Low-res Far Shadows" option to dynamically downscale far shadow map cascades (halving/quartering resolutions).
   • Saves rendering time and VRAM for distant shadow cascades.
   • Exposed via UI checkboxes in OpenBVE main settings, RouteViewer, and ObjectViewer.

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not done yet, but i will set this PR as open.