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 Job ID Name Coverage
  Containers Build
passed #516887
debian

00:00:21

 
  Build+Test
passed #516896
meson-arm64

00:02:34

passed #516895
meson-armhf

00:02:38

passed #516889
meson-clang

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passed #516893
meson-clover

00:10:09

passed #516897
meson-i386

00:03:35

passed #516892
meson-main

00:08:07

passed #516888
meson-swr-glvnd

00:06:15

passed #516894
meson-vulkan

00:03:08

passed #516899
scons-llvm

00:04:16

passed #516898
scons-nollvm

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passed #516890
scons-swr

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passed #516891
scons-win64

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

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passed #516901
test-llvmpipe-gles2 2/4

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passed #516902
test-llvmpipe-gles2 3/4

00:04:41

passed #516903
test-llvmpipe-gles2 4/4

00:04:05