Both gst_vulkan_image_buffer_pool_set_config() and
gst_vulkan_image_buffer_pool_alloc() functions share the same code to create
Vulkan images for different purposes.
This patch refactor them in a new helper function that creates the images and
stores them in a buffer if it's passed as output parameters, such as the
offsets.
This patch also adds specifics guards for Vulkan's symbols for better grained
API usage, but also for prepare_buffer() the guard is set where the symbol is
used.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/9492>