Default values for materials[REOPEN]

## Default variables for material Get method

Allow setting default variable, instead of using default hardcoded variablles.

Hardcoded variables feel very limiting, There should be way to override them.

## Solution

* Maybe overload Get method, with extra parameter with default.
* Boolean flag to enable defaults.
* add to *aiReturn* param missing option

Currently API seems to not support such option, which seems like oversight.

Start by locating the material Get method and the aiReturn parameter handling in the Assimp source. Compare the existing hardcoded default variables with the proposed override options, then define and document one consistent API behavior and tests that show callers can replace or disable those defaults.

Written by the indexing model from the issue text.