Give the fragment stage uniform blocks unique names in IBL shaders

The environment map, prefilter, and lightmap UV raster shaders declared
a uniform block named buf in both the vertex and fragment stage, with
different members. For OpenGL, the blocks are translated to plain
struct uniforms, so the linked program contained two conflicting
definitions of struct buf. NVIDIA accepts this, but the AMD driver
fails to link the program with "Fail to build reflection info", so no
environment map is generated from a light probe.

Rename the fragment stage block to buf2, matching its instance name.
The block in lightmapuvraster.frag had no instance name, so the GL
uniform was named after a SPIR-V id, which could end up clashing with
the vertex stage's similarly generated name. Give it the instance name
ubuf2, like in the other shaders.

Pick-to: 6.12
Fixes: QTBUG-151610
Change-Id: Ib9420591469b48fa0e8aa6424c2c8bdaa484b384
Reviewed-by: Jonas Karlsson <jonas.karlsson@qt.io>