Added Assimp sources to Overload

[adriengivry]

Description

Added assimp to the Overload solution, now building it from sources.

• Since Assimp is now also being built in Debug mode, assertions are enabled when flags are invalid/incompatible.
  • To address that, a FixFlags function has been added, so that flags passed to Assimp are validated.
  • In the future, we should update the asset properties for models to avoid hitting cases where flags are invalid.
• Updated default model flags to avoid invalid setups (i.e. OPTIMIZE_GRAPH cannot be set if PRE_TRANSFORM_VERTICES is set)
• Assimp version is still the same (5.4.3)

Review Guidance

• This assimp installation only includes the importers we use (OBJ & FBX). Adding support for other importers (or exporters), the necessary files can be added (c.f. https://github.com/assimp/assimp/tree/master/code/AssetLib). This keeps the dependency super lib (~5MB).
• Unused assimp's dependencies (in the contrib/ folder) have also been removed to keep the sources lightweight. Some importers/exporters might require adding back some dependencies to the contrib/ folder.

Related Issues

Related to #459

To-Do

• Fix invalid model parser flags assert (caught when building in debug mode, but flags were already invalid)
• Validate functionnalities