turnip: fix alpha to coverage in no color and unused attachment cases

In cases where the alpha coverage is enabled but the color attachment is
either unused or absent there should be a dummy mrt to make the draw behave
correctly.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Yannik Marek <yannik@marek.ca>
Part-of: <!8952>
55 jobs for !8952 with alpha_to_coverage in 13 minutes and 11 seconds (queued for 3 seconds)
latest detached
Status Job ID Name Coverage
  Sanity
passed sanity #7889381

00:00:09

 
  Container
passed arm_build #7889384
aarch64

00:00:17

passed arm_test-base #7889385

00:00:21

passed windows_build_vs2019 #7889386
windows shell 1809 mesa

00:00:11

passed x86_build-base #7889382

00:00:19

passed x86_test-base #7889383

00:00:13

 
  Container 2
passed android_build #7889394

00:00:16

passed arm64_test #7889397

00:00:14

passed armhf_test #7889398

00:00:16

passed i386_build #7889391

00:00:21

passed kernel+rootfs_amd64 #7889387

00:01:32

passed kernel+rootfs_arm64 #7889388
aarch64

00:01:04

passed kernel+rootfs_armhf #7889389
aarch64

00:00:12

passed ppc64el_build #7889392

00:00:15

passed s390x_build #7889393

00:00:15

passed x86_build #7889390

00:00:15

passed x86_test-gl #7889395

00:00:19

passed x86_test-vk #7889396

00:00:13

 
  Meson X86 64
passed meson-clang #7889404

00:01:54

passed meson-clover #7889405

00:02:56

passed meson-clover-testing #7889401

00:01:55

passed meson-gallium #7889402
gstreamer

00:01:55

passed meson-release #7889403

00:01:50

passed meson-testing #7889399

00:03:09

passed meson-testing-asan #7889400

00:02:35

passed meson-vulkan #7889406

00:01:54

 
  Build Misc
passed meson-android #7889407

00:01:49

passed meson-arm64 #7889409
aarch64

00:01:39

passed meson-arm64-asan #7889410
aarch64

00:03:08

passed meson-arm64-build-test #7889411
aarch64

00:01:37

passed meson-armhf #7889408
aarch64

00:01:46

passed meson-i386 #7889412

00:03:31

passed meson-mingw32-x86_64 #7889415

00:01:58

passed meson-ppc64el #7889414
kvm

00:01:50

passed meson-s390x #7889413
kvm

00:02:41

 
  Freedreno
passed a306_gles2 #7889416
google-freedreno-db410c

00:04:43

passed a306_gles3 1/3 #7889417
google-freedreno-db410c

00:08:49

passed a306_gles3 2/3 #7889418
google-freedreno-db410c

00:08:49

passed a306_gles3 3/3 #7889419
google-freedreno-db410c

00:08:46

passed a306_gles3_options #7889420
google-freedreno-db410c

00:02:36

passed a530_gles2 #7889421
google-freedreno-db820c

00:06:08

passed a530_gles31 1/2 #7889424
google-freedreno-db820c

00:06:15

passed a530_gles3 1/2 #7889422
google-freedreno-db820c

00:08:24

passed a530_gles31 2/2 #7889425
google-freedreno-db820c

00:06:06

passed a530_gles3 2/2 #7889423
google-freedreno-db820c

00:07:57

passed a630_gl #7889429
google-freedreno-cheza

00:01:20

passed a630_gles2 #7889426
google-freedreno-cheza

00:01:32

passed a630_gles3 #7889428
google-freedreno-cheza

00:04:32

passed a630_gles31 #7889427
google-freedreno-cheza

00:03:56

passed a630_gles_asan #7889431
google-freedreno-cheza

00:05:04

passed a630_gles_others #7889430
google-freedreno-cheza

00:02:27

passed a630-traces #7889435
google-freedreno-cheza

00:07:01

passed a630_vk 1/2 #7889432
google-freedreno-cheza

00:06:46

passed a630_vk 2/2 #7889433
google-freedreno-cheza

00:08:03

passed a630_vk_sysmem #7889434
google-freedreno-cheza

00:02:10