freedreno/a6xx: sample-shading support

Enables:

  OES_sample_shading
  OES_sample_variables
  OES_shader_multisample_interpolation

Signed-off-by: Rob Clark <robdclark@chromium.org>
14 jobs for !739 with wip/sample-shading in 12 minutes and 13 seconds (queued for 5 seconds)
latest detached
Status Job ID Name Coverage
  Containers Build
passed #263038
debian

00:00:22

 
  Build+Test
passed #263040
meson-clang

00:11:47

passed #263046
meson-gallium-clover-llvm

00:05:15

passed #263047
meson-gallium-clover-llvm39

00:05:53

passed #263045
meson-gallium-drivers-other

00:08:06

passed #263044
meson-gallium-radeonsi

00:04:40

passed #263048
meson-gallium-st-other

00:06:19

passed #263039
meson-gallium-swr

00:09:11

passed #263043
meson-glvnd

00:05:00

passed #263042
meson-loader-classic-dri

00:06:47

passed #263041
meson-vulkan

00:03:55

passed #263050
scons-llvm

00:04:15

passed #263049
scons-nollvm

00:06:00

passed #263051
scons-swr

00:05:53