The major functionality gain this provides is proper reference counting
for a descriptor set. Overall this allows us to create descriptor sets
when they are needed (or reused from a cache) without violating any of
vulkan's object synchronisation requirements.
As there are a fixed number of sets available in a pool, the number of
descriptors in elements is currently hardcoded to 32. This can be extended
in a future change to create pools on the fly if that limit is ever overrun.
Allows a cleaner control flow when there is no fence available for use
with the trash list. An always signalled fence type will always return
TRUE for gst_vulkan_fence_is_signalled.
clamp-to-border will return the border color which is typically black,
white or transparent. When linear filtering the edge pixels will
typeically be combined with the border color which is not typically what
we want. Especially when color converting, this removes a green box
around the edge when converting YUV->RGB.
