glx: Implement GLX_EXT_context_priority

This is the GLX counterpart to EGL_IMG_context_priority, so this is
pretty straightforward. Add the extension to the list in libGL and teach
the DRI code how to translate the attribute.
22 jobs for !902 with glx-ext-context-priority in 13 minutes and 59 seconds (queued for 4 seconds)
latest detached
Status Job ID Name Coverage
  Containers Build
passed #526100
debian

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  Build+Test
passed #526117
meson-arm64

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passed #526116
meson-armhf

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passed #526104
meson-clang

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passed #526112
meson-clover

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passed #526118
meson-i386

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passed #526110
meson-main

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passed #526102
meson-swr-glvnd

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passed #526114
meson-vulkan

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passed #526120
scons-llvm

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passed #526119
scons-nollvm

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passed #526106
scons-swr

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passed #526108
scons-win64

00:06:30

 
  Test
passed #526121
test-llvmpipe-gles2 1/4

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passed #526122
test-llvmpipe-gles2 2/4

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passed #526123
test-llvmpipe-gles2 3/4

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passed #526124
test-llvmpipe-gles2 4/4

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passed #526125
test-softpipe-gles2 1/4

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passed #526126
test-softpipe-gles2 2/4

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passed #526127
test-softpipe-gles2 3/4

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passed #526128
test-softpipe-gles2 4/4

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passed #526129
test-softpipe-gles3-limited

00:02:31