turnip: VK_EXT_descriptor_buffer

Now that the inline uniform rewrite has landed, this is relatively straightforward to implement. We just need to tweak the descriptor writing slightly to match the API of CmdGetDescriptorEXT, pull out the part where we actually emit the descriptor sets, and implement CmdSetDescriptorBufferOffsetsEXT very similarly to CmdBindDescriptorSets. Embedded samplers are handled by creating the descriptor set up-front when the layout is created and just binding it, although we could improve it later by suballocating these if necessary.