radeonsi: Use TRUNC_COORD on samplers

radeonsi: Use TRUNC_COORD on samplers

The default behaviour (0) is: "round-nearest-even to n.6 and drop fraction when point sampling" whereas the OpenGL spec simply wants us to floor it (1) "truncate when point sampling".
See 8.14.2 in the OpenGL spec:

https://www.khronos.org/registry/OpenGL/specs/gl/glspec46.core.pdf

The Direct3D spec also mandates this (https://microsoft.github.io/DirectX-Specs/d3d/archive/D3D11_3_FunctionalSpec.htm#7.18.7%20Point%20Sample%20Addressing)

On WineD3D:
This fixes some point-sampling texture precision issues in some Direct3D 9 titles such as Guild Wars 2 and htoL#NiQ: The Firefly Diary that are not present on other vendors.

CC: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3953>