Joint indexes is limited inside one skeleton element

Inside a `Skeleton` element there is usually a structure a `Joints`, each `Joint` having one unique index.

Having a `Joint` with an index too high results in OpenGL error. (Observed value for maximal index are 255 and 144, depending on context, see how to reproduce below)

The error message starts with

Requesting OpenGL 4.3 core context succeeded
Failed to link shader program: Vertex info

The full error message can be found here on pastebin.

How to reproduce, method 1:

• Open "Qt Quick 3D - Simple skinning exemple"
• Look for SimpleSkinning.qml, line 122 (i.e. `index: 0`)
• Replace the 0 with 254 : the application runs without error. (The display looks a little distorted, because there isn't 254 inverseBindPoses matrices, but a default identity matrix is used)
• Replace the 0 with 255 : the mesh is not displayed and a runtime error is produced, comming from OpenGL

How to reproduce, method 2:

• Get the files in attachement
• The "MagicNumber143" element contains 143 joints element and runs.
• Comment line 30 and uncomment line 31 of main.qml : The "MagicNumber144" element contains 144 joints and will trigger the error