Fix priority crash, import, export, history and opacity paths

[Loggableim]

Summary

Draft patch for the priority issue triage. The changes add defensive validation at the boundaries where malformed config, missing GPU/file data, failed layer creation, and history traversal previously reached unchecked dereferences.

Issue coverage

• UI scale accepts negative values causing app to not start - locale issue with comma vs decimal point #2082: validate UI scale on config load, import, save, and UI application; invalid values fall back to 1.0.
• Brush opacity can exceed 1.0 causing weird painting behavior #2084: clamp brush opacity after node, pressure, and cursor multipliers.
• Crash when inporting blender project #2052: write Blender's temporary OBJ to the user-writable internal save path, remove stale output, and stop cleanly when Blender produces no OBJ.
• Crash when saving packed project #2065: make packed-asset export report missing/unloaded assets and abort before writing a partial packed project.
• Crashing when trying to export #2080: validate export presets and layer inputs before texture export; provide a generic preset fallback for missing/corrupt preset data.
• Crash #2090: keep 2D touch-view input safe when the active layer has no paint texture.
• Undo-Redo-Mechanism issues #792: restore masks from the same layer-delete operation together during undo and bound redo scanning; avoid using deleted layer pointers in the UI delete path.
• Crash when adding Fill mask. #2062: make fill-mask/layer conversion and maximum-layer creation failure-safe, including sculpt-layer creation.

Verification

• python paint/tests/test_priority_fixes.py -> 6 regression checks passed.
• GCC -fsyntax-only passed for all changed C translation units.
• paint\\..\\base\\make.bat --help generated the Windows project files and compiled all bundled shaders successfully.
• The native MSBuild attempt is currently blocked by the environment: Visual Studio C++ targets (Microsoft.Cpp.Default.props / MSBuild.exe) are not installed. Linux/Vulkan and GPU-specific issue reproduction was not possible in this Windows checkout.

This is intentionally a draft so the upstream maintainer can review the platform-specific paths and run the native Linux/Windows runtime scenarios.