softpipe: Add support for ARB_derivative_control

Enables and passes piglits:

spec/ARB_drivative_control/
        dfdx-coarse
        dfdx-dfdy
        dfdx-fine
        dfdy-coarse
        dfdy-fine

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
17 jobs for master in 22 minutes and 2 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Containers Build
passed #503228
debian

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  Build+Test
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meson-arm64

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meson-armhf

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passed #503230
meson-clang

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passed #503234
meson-clover

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passed #503238
meson-i386

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passed #503233
meson-main

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passed #503229
meson-swr-glvnd

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passed #503235
meson-vulkan

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passed #503240
scons-llvm

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passed #503239
scons-nollvm

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passed #503231
scons-swr

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passed #503232
scons-win64

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  Test
passed #503241
test-llvmpipe-gles2 1/4

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passed #503242
test-llvmpipe-gles2 2/4

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passed #503243
test-llvmpipe-gles2 3/4

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passed #503244
test-llvmpipe-gles2 4/4

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