iris: support dmabuf imports with offsets

this adds support for imports where the image data begins at an offset
from the start of the buffer, as used in h/x264

fixes kwg/mesa#47
14 jobs for !770 with offset in 8 minutes and 42 seconds (queued for 4 seconds)
latest detached
Status Job ID Name Coverage
  Containers Build
passed #271855
debian

00:00:17

 
  Build+Test
passed #271857
meson-clang

00:08:22

passed #271863
meson-gallium-clover-llvm

00:04:04

passed #271864
meson-gallium-clover-llvm39

00:03:27

passed #271862
meson-gallium-drivers-other

00:04:45

passed #271861
meson-gallium-radeonsi

00:03:33

passed #271865
meson-gallium-st-other

00:03:53

passed #271856
meson-gallium-swr

00:05:40

passed #271860
meson-glvnd

00:03:58

passed #271859
meson-loader-classic-dri

00:04:00

passed #271858
meson-vulkan

00:02:59

passed #271867
scons-llvm

00:05:03

passed #271866
scons-nollvm

00:04:47

passed #271868
scons-swr

00:07:16