Fix passing node count to oit shader in buffer mode

The qt_listNodeCount is passed to the shader using the texture size
calculation in buffer mode when it should be fed directly. This can lead
the shader to write over the buffer bounds in heavy load.

Pick-to: 6.12
Change-Id: Ia61af7292b314ae1ce10cef20ebe3d0e1a698a03
Reviewed-by: Andy Nichols <andy.nichols@qt.io>