tests rely on undefined behavior

[KonstantinSeurer]

Float to integer conversions

The SPIR_V spec states:

Khronos issue #491: For OpConvertFToU and OpConvertFToS, behavior is undefined if Result Type is not wide enough to hold the converted value.

test_shader_instructions expects the conversions to saturate and map NAN to 0. The issue can be reproduced when running on lavapipe.

Texture sampling level selection

The Vulkan spec allows lod=0.5 to be rounded to 1 or 0: https://registry.khronos.org/vulkan/specs/1.3-extensions/html/vkspec.html#textures-image-level-selection with 0 being preferred. test_sample_instructions test 23 expects it to be ceiled.

[doitsujin]

As discussed on Discord:

• Correct ftou/ftoi behaviour would require us to open-code the conversion instructions with at least three OpFCompare+OpSelect, which drivers for hardware that already behaves this way would have to optimize away. This is kind of problematic for common operations like this.
• Test 23 essentially tests for D3D behaviour, which matches the "alternative" mode in the Vulkan spec. As it stands, we do not have enough control over samplers to enforce this.

[KonstantinSeurer]

Makes sense. I will se what I can do on the lavapipe side.