mesa: remove snprintf macro in imports.h to fix MSVC build

mesa: remove snprintf macro in imports.h to fix MSVC build

snprintf is a macro in the MSVC stdio.h header and we needed to
include that header before imports.h where we also defined an
snprintf macro.  Otherwise, the MSVC build would fail.  The recent
mtypes.h removal patches seems to have exposed this issue.

This patch simply removes our snprintf macro and replaces one use
of it in teximage.c with _mesa_snprintf().  There are other calls
to snprintf() in DRI drivers, but none of them are built on Windows.

Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Reviewed-by: Neha Bhende <bhenden@vmware.com>