virgl: Fix a strict-aliasing violation in the encoder

virgl: Fix a strict-aliasing violation in the encoder

As per the C spec, it is illegal to alias pointers to different
types. This results in undefined behaviour after optimization
passes, resulting in very subtle bugs that happen only on a
full moon..

Use a memcpy() as a well defined coercion between the double
to uint64_t interpretations of the memory.

V.2: Use static_assert() instead of assert().
V.3: Use C99 compat STATIC_ASSERT() over C11 static_assert().

Signed-off-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Acked-by: Dave Airlie <airlied@redhat.com>