v3d/compiler: fix V3D double-rounding of .8 fixed-point XY coordinates

Pre-V3D 4.3 hardware has a quirk where it expects XY coordinates in .8 fixed-point format, but then it will internally round it to .6 fixed-point, introducing a double rounding. The double rounding can cause very slight differences in triangle raterization coverage that can actually be noticed by some CTS tests.

The correct fix for this as recommended by Broadcom is to convert to .8 fixed-point with ffloor().

Fixes: dEQP-VK.renderpass.suballocation.subpass_dependencies.late_fragment_tests.*