softpipe: evaluate cube the faces on a per sample bases

Now that the LOD is evaluated up front the cube faces can also be
evauate on a per sample basis instead of using the quad.

This fixes a large number of deqp gles 3 and 31 cube texture tests.

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
14 jobs for !702 with softpipe-use-explicite-gradients in 9 minutes and 9 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Containers Build
passed debian #272402

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  Build+Test
passed meson-clang #272404

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passed meson-gallium-clover-llvm #272410

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passed meson-gallium-clover-llvm39 #272411

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passed meson-gallium-drivers-other #272409

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passed meson-gallium-radeonsi #272408

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passed meson-gallium-st-other #272412

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passed meson-gallium-swr #272403

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passed meson-glvnd #272407

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passed meson-loader-classic-dri #272406

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passed meson-vulkan #272405

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passed scons-llvm #272414

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passed scons-nollvm #272413

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passed scons-swr #272415

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