Protocol

 - finalize docs/VK_EXT_command_serialization.md
   - pNext should use depth-first encode
 - finalize xmls/VK_EXT_command_serialization.xml
 - finalize xmls/VK_MESA_venus_protocol.xml
 - verify the correctness of the generated code

Features

 - improve robustness for the renderer
 - write tests
 - clean up the generator
 - do not generate dead code
 - easier to use APIs for the renderer and the driver

Performance

 - help compiler generate better code (always_inline, macros)
 - rework templates/driver_calls.h
 - add smartness to some commands (e.g., ignore some members of
   VkWriteDescriptorSet according to VkDescriptorType)
