draw: Fix type mismatch between draw_private.h and LLVM

draw: Fix type mismatch between draw_private.h and LLVM

draw_vertex_buffer declared the size field to be a size_t, but the LLVM
code used an int32 instead.  This caused problems on big-endian 64-bit
targets, because the first 32-bit chunk of the 64-bit size_t was always 0.

In one sense size_t seems like a good choice for a size, so one fix
would have been to try to get the LLVM code to use the equivalent of
size_t too.  However, in practice, the size is taken from things like ~0
or width0, both of which are int-sized, so it seemed simpler to make the
size field int-sized as well.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Richard Sandiford <rsandifo@linux.vnet.ibm.com>