libs: videopool: fix undocumented behavior and counting
This is a (modified) cherry-pick of !100 (closed)
CC: @He_Junyan
gst_vaapi_video_pool_reserve_unlocked() hit an undocumented behavoir because it locks twice the same mutex.
Also, n had different meanings in the current code: as an increase value and as a new total of allocated surfaces.
This patche removes the undocumented behavoir (usually a deadlock) and fixes the meaning of n as the new total of allocated surfaces.
Signed-off-by: Víctor Manuel Jáquez Leal vjaquez@igalia.com