Add a cosmetic change to replace VAAPI buffer with VA buffer and most
importantly fix warnings spitted out during build on 64-bit platforms.

../../tests/simple-encoder.c:211:5: warning: format ‘%d’ expects argument of type ‘int’, but argument 4 has type ‘gssize’ [-Wformat=]
     g_warning ("Invalid VAAPI buffer size (%d)", size);
     ^
../../tests/simple-encoder.c:217:5: warning: format ‘%d’ expects argument of type ‘int’, but argument 4 has type ‘gssize’ [-Wformat=]
     g_warning ("Failed to create output buffer of size %d", size);
     ^