Qt3DRender::QTexture2D does not handle opacity. Therefore, it is not
possible to use the default Qt3DExtras::QDiffuseSpecularMaterial
implementation. This problem is solved by using
Qt3DRender::QMaterial and copying the
Qt3DExtras::QDiffuseSpecularMaterial shaders from Qt3D source
code. Then, the texture color needs to be changed to set the correct
opacity. This is achieved in the fragement shader:
```
vec4 diffuseTextureColor = vec4(texture(diffuseTexture, texCoord).rgb,
opacity);
```
instead of the default:
```
vec4 diffuseTextureColor = vec4(texture(diffuseTexture, texCoord));
```
As far as the FrameGraph is concerned, this is already handled in
Qgs3DMapScene::finalizeNewEntity which checks for all material which
have an effect with an opacity parameter.
This is a "null" shading material which should be used when
rendering models or scenes with native textures and no material
should be explicitly set on the loaded entities.
