Intialize memory to zero to silence MSan.

Intialize memory to zero to silence MSan.

The MemorySanitizer tool can't instrument JIT-compiled code, so it's
unaware that the vertex processing routine writes the clip flags before
they're being read by triangle setup. This false positive can be
silenced by zeroing the memory at allocation. For good measure, zero
out all intermediate buffers.

Bug chromium:737875

Change-Id: Ic37ff5c64cb63bbddb151744af1d7dff0a254c2d
Reviewed-on: https://swiftshader-review.googlesource.com/10431
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>