anv: Support B8G8R8A8_UNORM storage images

Even though B8G8R8A8_UNORM isn't one of the classic storage image
formats, it is the usual swapchain image format on Linux so it's helpful
to support it for storage.  Because SPIR-V provides no enums for BGRA
formats, it can only be used without a format which, on ANV, means it
must be a write-only image.  However, since the intended use-case is for
writing to a swapchain image from a compute shader, this doesn't seem
onerous.
31 jobs for !2537 with review/anv-bgra-storage in 21 minutes and 11 seconds (queued for 1 minute and 51 seconds)
latest detached
Status Job ID Name Coverage
  Container
passed #1987606
aarch64
arm_build

00:00:21

passed #1987608
aarch64
arm_test

00:00:21

passed #1987599
x86_build

00:00:22

passed #1987604
x86_build_old

00:00:26

passed #1987600
x86_test-gl

00:00:17

passed #1987602
x86_test-vk

00:00:16

 
  Meson X86 64
passed #1987616
meson-clang

00:03:56

passed #1987614
meson-classic

00:02:03

passed #1987617
meson-clover

00:04:32

passed #1987619
meson-clover-old-llvm

00:02:54

passed #1987612
meson-gallium

00:03:24

passed #1987611
meson-testing

00:02:08

passed #1987621
meson-vulkan

00:02:15

 
  Scons
passed #1987625
scons

00:06:06

passed #1987626
scons-old-llvm

00:02:50

passed #1987623
scons-win64

00:09:19

 
  Meson Misc
passed #1987630
aarch64
meson-arm64

00:02:55

passed #1987632
aarch64
meson-arm64-build-test

00:02:44

passed #1987628
aarch64
meson-armhf

00:02:51

passed #1987634
meson-i386

00:06:05

passed #1987639
meson-mingw32-x86_64

00:04:49

passed #1987637
meson-ppc64el

00:02:48

passed #1987636
meson-s390x

00:05:27

 
  Freedreno
passed #1987641
google-freedreno-db410c
arm64_a306_gles2

00:08:55

passed #1987643
google-freedreno-db820c
arm64_a530_gles2

00:09:51

passed #1987644
google-freedreno-db820c
arm64_a530_gles3 1/6

00:08:42

passed #1987646
google-freedreno-db820c
arm64_a530_gles3 2/6

00:08:55

passed #1987648
google-freedreno-db820c
arm64_a530_gles3 3/6

00:08:27

passed #1987653
google-freedreno-db820c
arm64_a530_gles3 4/6

00:17:05

passed #1987655
google-freedreno-db820c
arm64_a530_gles3 5/6

00:08:36

passed #1987657
google-freedreno-db820c
arm64_a530_gles3 6/6

00:06:46