fix: voxel cache rendering corruption

[Belonit]

Summary

Fix several independent correctness problems in voxel rendering that are
exposed by optimized clang-cl builds and can corrupt cached images.

Expected result: every cached voxel image contains only pixels produced for
that render, without coloured artifacts around voxel units.

The four fixes are:

• Harden RLE compression by stopping transparent runs at the input boundary
  and sizing its temporary buffer for a worst-case voxel row.
• Correct adjacent-pixel writes so the second pixel uses the complete next
  packed-buffer index.
• Remove undefined fixed-point conversions by converting through int before
  wrapping to unsigned 8.8 coordinates.
• Clear the complete colour and depth work buffers before each voxel render,
  removing unreliable previous-region tracking and the now-unused partial-clear
  helpers. The performance impact on modern systems is minimal because one
  contiguous memset is better optimized than many row-by-row calls.

Behavior and compatibility

Bug fix. Maps, rules, saved games, replays, network data formats, deterministic
simulation, and COM/ABI surfaces are unchanged. The change affects only the
temporary voxel rendering cache; no migration is required.

Validation

• The experimental Linux clang-cl cross-build completed successfully.
• Runtime check: reproduce a voxel cache render after a prior differently sized
  render and confirm that the coloured artifact is absent.
• The supported Visual Studio 2022 Win32 Debug and Release builds were not run.

Documentation

No documentation change is needed. These are internal rendering correctness
fixes that do not change a documented interface or workflow.

Checklist

• The change is focused; unrelated mechanical cleanup is separate
• Compatibility effects and any migration are explicit
• Documentation impact is stated
• Validation distinguishes what passed, failed, and was not run
• No prohibited assets, binaries, SDKs, credentials, or generated output are included

[github-actions]

Development builds of cd5f29c:

• opents-Release-09a2250
• opents-Debug-09a2250

The links work without a GitHub account. Artifacts expire after 90 days, and this comment follows the latest successful build.

[Belonit]

Updated after identifying the root cause: undefined conversions in the 8.8 fixed-point coordinate calculations.