mesa/program: Take ARB_framebuffers_no_attachments into account in wpos correction

If a drawbuffer is an fbo without an attachment then its 'Height' will be zero,
and we have to take its 'DefaultGeometry.Height' into account.

Fixes on softpipe (with the exception of tests that use multisample):
  dEQP-GLES31.functional.fbo.no_attachments.*

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
17 jobs for !1711 with softpipe-fb-noattachment in 15 minutes and 27 seconds (queued for 3 seconds)
detached
Status Name Job ID Coverage
  Containers Build
passed debian #516887

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  Build+Test
passed meson-arm64 #516896

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passed meson-armhf #516895

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passed meson-clang #516889

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passed meson-clover #516893

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passed meson-i386 #516897

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passed meson-main #516892

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passed meson-swr-glvnd #516888

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passed meson-vulkan #516894

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passed scons-llvm #516899

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passed scons-nollvm #516898

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passed scons-swr #516890

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passed scons-win64 #516891

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  Test
passed test-llvmpipe-gles2 1/4 #516900

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passed test-llvmpipe-gles2 2/4 #516901

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passed test-llvmpipe-gles2 3/4 #516902

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passed test-llvmpipe-gles2 4/4 #516903

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