meson: Enable -Werror=int-conversion.

meson: Enable -Werror=int-conversion.

I think implicit conversions here are almost always wrong:

1) wrong argument position ptr vs. int
2) will often have issues with 32-bit platforms.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/2570>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/2570>