Make dynamic shadows load and render correctly

[Rokk001]

Dynamic shadows could fail while loading shader resources, lose custom shader passes, cover only part of the visible ground or disappear on small creatures.

Change

• Register the required renderer media in the internal resource group.
• Preserve custom shader passes with the integrated additive shadow path.
• Fit shadow coverage to the visible ground and keep receiver, caster and ambient-light behavior consistent across custom and generated materials.
• Preserve shadows for small creatures.

Coordination

Depends on #47 for the Windows-tested build stack. No matching open issue was found. PRs #41 and #45 touch the same renderer area for separate visual changes, while PR #15 changes the renderer baseline.

Verification

Focused resource, shader-pass, receiver, settings and lighting checks passed during development. The reconstructed stack through #81 completed a clean Windows x64 Release build, and the user accepted the resulting startup and shadow behavior. Other platforms remain unverified.

Review only the shadow changes

[tomluchowski]

Just checked --- it's all nice but creatures most of the time DO NOT cast any shadows and most of the potential surfaces where the shadow should be cast don't show anything -- the shadow simply does not appear. Please try for that example 'Forgotten Treasures' , dig a large room and see how it behaves.... Anyway you are welcome to help in here, please remember you can always show up at our discord channel, it's link is mentioned in several places...

[Rokk001]

Thanks for testing - this PR addresses the crashes, but the missing shadows in your Forgotten Treasures example still need investigation.

[Rokk001]

I've pushed fixes for missing creature shadows and excessive darkening; the Windows build and rendering checks pass, but Forgotten Treasures still needs a manual retest.