Added support for "map_Bump -bm"

[TerenceRussell]

Hi,

I've added Obj model support for the bump map -bm #### multiplier. I've attempted to follow the gltf use of "$tex.scale" as a guide for making it available.

An example Obj line with the parameter:
map_Bump -bm 0.398056 textures\main_normal.png

These changes will load the 0.398056 factor into a material property for either Normal or Height maps (ie: Obj map_Bump). The property is only set if the scalar is not 1.0.

The property can be retrieved like so:
aiMaterial* mtl = scene->mMaterials[m];
ai_real bm = 1.0;
aiReturn rc = aiReturn_SUCCESS;

if( mtl->GetTexture(aiTextureType_NORMALS, texIndex, &path) == AI_SUCCESS) {
rc = aiGetMaterialFloat(mtl, AI_MATKEY_OBJ_BUMPMULT_NORMALS(0), &bm);
}
else if(mtl->GetTexture(aiTextureType_HEIGHT, texIndex, &path) == AI_SUCCESS) {
rc = aiGetMaterialFloat(mtl, AI_MATKEY_OBJ_BUMPMULT_HEIGHT(0), &bm);
}

[kimkulling]

Merged, thanks a lot for your contribution.