Unify behavior for skinned and not skinned meshes.

In current implementation processing and rendering of skinned and not skinned meshes implemented in different ways. This approach has drawbacks:

• 2 materials should be created for skinned and not skinned meshes
• 2 shader pairs should be created for skinned and not skinned meshes
• whenever a mesh is modified it should be analyzed for whether it is weighted or not to set correct value to the "skeleton" property.
  That means that in the c++ code there should be a loop through all the vertices which is ran whenever the mesh changes.
• semi-skinned meshes are not supported. If summary weight of a vertex is less than 1.0, the vertex position is incorrect.

I suggest that the approach for rendering skinned and not skinned meshes should be unified.
To do that this vertex shader can be used:

    vec4 pos = vec4(VERTEX, 1.0);

    float totalWeight = 0.0;
    vec4 weightedPos = vec4(0.0, 0.0, 0.0, 0.0);

    for(int i = 0; i != 4; ++i)
    {
        weightedPos += BONE_TRANSFORMS[JOINTS[i]] * WEIGHTS[i] * pos;
        totalWeight += WEIGHTS[i];
    }

    if(totalWeight < 1.0)
    {
        weightedPos += MODEL_MATRIX * (1.0 - totalWeight) * pos;
    }

    POSITION = PROJECTION_MATRIX * (VIEW_MATRIX * weightedPos);

With this shader code:

• when total weigh is 1.0 the vertex position completely depends on the joint matrices.
• If it is between 0.0 and 1.0, depending on the total weight value vertex position is partially weighted to MODEL_MATRIX.
• If totalWeight is 0 i.e. the mesh is not skinned, it is fully weighted to the node global transform - MODEL_MATRIX.

Benefits of this approach:

• only one material is required for skinned/semi-skinned and not skinned meshes.
• only one pair of shaders is required for skinned/semi-skinned and not skinned meshes.
• skeleton property can always be set to the model, no need to loop through all the vertices to check whether the mesh is weighted or not.
• correct rendering of semi-weighted meshes.

In order to implement this approach further changes should be made:

• MODEL_MATRIX should not be set to identity when the skeleton property is not null. To not break current implementation, MODEL_MATRIX_SKINNED with correct values could be added and used instead.
• the shader should compile if there are no Joint nodes in the scene (https://bugreports.qt.io/browse/QTBUG-101076).