Using `local_size_x = 512` in the shader used in the Vulkan interop
tests fails on older hardware because this exceeds the maximum barrier
size limit.
Changed to 256, which is still a multiple of 32 and 64,
and is below the hardware limitations. The tests are not performance
tests anyways, so the performance penalty, if any, is acceptable.
Signed-off-by: Ahmed Hesham <ahmed.hesham@arm.com>
8b1a44f93f