Remove unnecessary memory barrier (#42)
Remove unnecessary memory barrier (#42)

RUN_GRAPHICS and RUN_COMMUTE had been designed with memory barriers at
the start, to ensure effects from prior commands had been
committed. Instead, this change puts a memory barrier at the end of
RUN_COMPUTE to ensure that its effects have been committed, and omits
the barrier before the commands. This avoids a memory barrier in
RUN_GRAPHICS, which is not supported for older API versions.

Fixes #35.
google/shadertrap,google/shadertrap,google/shadertrap,google/shadertrap,google/shadertrap