iris: Properly allow rendering to RGBX formats.

I was converting them at pipe_surface creation time, but not when
answering queries about whether formats support rendering.  This caused
a lot of FBO incomplete errors for formats that ought to be supported.

Fixes "Child of Light", which uses PIPE_FORMAT_R8G8B8X8_UNORM_SRGB.

Also fixes Witcher 1 using wined3d (GL) according to Timur Kristóf.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109738
24 jobs for !298 with iris-childoflight in 37 minutes and 53 seconds (queued for 23 minutes and 30 seconds)
detached
Status Job ID Name Coverage
  Build+Test
passed #137116
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

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passed #137111
build:meson-gallium-st-other

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passed #137105
build:meson-gallium-swr

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build:meson-glvnd

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failed #137103
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

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passed #137125
build:scons-swr

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Name Stage Failure
failed
build:meson-vulkan Build+Test
Removing src/util/xmlpool/Makefile.in
Removing src/vulkan/Makefile.in
HEAD is now at ef17d1d2c61 iris: Properly allow rendering to RGBX formats.
Checking out b45186a6 as iris-childoflight...
fatal: reference is not a tree: b45186a6cd0ed7539e427d4443671faef9f95058
Uploading artifacts...
WARNING: untracked: no files
ERROR: No files to upload
ERROR: Job failed: exit code 1
failed
build:meson-loader-classic-dri Build+Test
Running on runner-78559e26-project-181-concurrent-1 via gst-gitlab-htz-runner1...
Fetching changes...
HEAD is now at ef17d1d2c61 iris: Properly allow rendering to RGBX formats.
Checking out b45186a6 as iris-childoflight...
fatal: reference is not a tree: b45186a6cd0ed7539e427d4443671faef9f95058
Uploading artifacts...
WARNING: untracked: no files
ERROR: No files to upload
ERROR: Job failed: exit code 1