Fix anonymous push constant blocks for OpenGL

Does not truly matter for previous patches since only Vulkan,
D3D12, and Metal are in scope for push constant support in the
first phase, but let's prepare for possible emulation of
push constants on OpenGL and D3D11 too.

Task-number: QTBUG-145700
Change-Id: I83ba48ca00cdddc506406471c52986ebc0774bf6
Reviewed-by: Andy Nichols <andy.nichols@qt.io>