modetest: make middle SMPTE colors transparent

This enables us to check for overlay planes which are located
'below' the primary plane.

Since the alpha value only has an effect when creating surfaces
with an alpha-pixelformat this doesn't affect the regular
XRGB8888 primary surface.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Acked-by: Emil Velikov <emil.l.velikov@gmail.com>