First attempt to refactor build_gles2_cmd_buffer.py:
1. Moves FUNC_INFO from build_gles2_cmd_buffer.py to function_info.json;
2. Moves I/O classes to writer.py
3. Moves Function related classes to functionutils.py
4. Creates a new class called Generator, it implements the common generation
code that can be used by opengl and in the future opencl.