gallium/aux: add PIPE_CAP_MAX_VARYINGS to u_screen

Allows drivers using `u_pipe_screen_get_param_defaults` to use a
fallback value for the new pipe cap. Default value of 8 based on GL 2.1
MAX_VARYING_FLOATS

Reviewed-by: Eric Anholt <eric@anholt.net>
25 jobs for max_varyings
Status Job ID Name Coverage
  Containers Build
canceled #115034
containers:ubuntu

 
  Build+Test
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build:make-gallium-drivers-other

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build:make-gallium-drivers-radeonsi

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build:make-gallium-drivers-swr

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build:make-gallium-st-clover-llvm-39

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build:make-gallium-st-clover-llvm-4

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build:make-gallium-st-clover-llvm-5

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build:make-gallium-st-clover-llvm-6

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build:make-gallium-st-clover-llvm-7

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build:make-gallium-st-other

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build:make-loader-classic-dri

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build:make-vulkan

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build:meson-gallium-clover-llvm5

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build:meson-gallium-clover-llvm6

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build:meson-gallium-clover-llvm7

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build:meson-gallium-drivers-other

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build:meson-gallium-radeonsi

canceled #115044
build:meson-gallium-st-other

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build:meson-gallium-swr

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build:meson-glvnd

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build:meson-loader-classic-dri

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build:meson-vulkan

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build:scons-llvm

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build:scons-nollvm

canceled #115058
build:scons-swr