Add flag for scaling models

[kenkit]

This fix should go to assimp so that scaling can also help reduce the mesh size generated.
As seen on some Opengl code here

void recursive_render (const struct aiScene *sc, const struct aiNode* nd, float scale)
{
	unsigned int i;
	unsigned int n=0, t;
	aiMatrix4x4 m = nd->mTransformation;

	aiMatrix4x4 m2;
	aiMatrix4x4::Scaling(aiVector3D(scale, scale, scale), m2);
	m = m * m2;

	// update transform
	m.Transpose();