Include Intel VA drivers on amd64, i386 architectures
As requested on the snapcraft forum, Intel's integrated graphics chips require an additional package va-driver-all
to get full hardware acceleration.
Add this as an architecture-specific stage-package for the wpewebkit part. While its dependency mesa-va-drivers
is available on ARM architectures as well, hardware acceleration works fine on Raspberry Pi and we don't want to bloat the snap further right now.