tests/kms_cursor: use safe values for test image to avoid rounding errors

instead of 0.5 use 1.0 for white color, this is due to bit replication
effect. Also set cairo to use same blending mode as what HW uses.

Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>