Includes a new GstVulkanHandlePool base class for pooling different
resources togther. The descriptor cache object is ported to
GstVulkanHandlePool with the exact same functionality.
A new GstVulkanFenceCache is also implemented for caching fences
which is used internally by GstVulkanDevice for creating or reusing
fences.
The existing GstVulkanTrashFenceList object now caches trash objects.
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.
