V4L/DVB (5736): Add V4L2_FBUF_CAP/FLAG_LOCAL/GLOBAL_INV_ALPHA

Michael Schimek requested the addition of inverted alpha framebuffer
caps/flags to support such hardware.
'Normal' alpha uses this formula to mix the framebuffer and video:
output = fb pixel * fb alpha + video pixel * (1 - fb alpha)
and the 'inverted' alpha uses this formula:
output = fb pixel * (1 - fb alpha) + video pixel * fb alpha

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>