Uniforms not updated when property is dynamically added to QShaderData

When a property is dynamically added to an existing QShaderData, the corresponding uniforms are not updated.

This seems to be a regression in 5.14; we have some code that relied on this which used to work with 5.12.

I have added a simple test case showing the problem. There's a shader that has a QShaderData parameter. When a timer is triggered, a property is added to the QShaderData, but the uniform is not updated.