Add Dear ImGUI submodule, refactor host to use it

* Add host-gl.{c,h}: these have the gl extension function pointers, and
  utilities for compiling shaders.
* Add host-ui.h, host-ui-simple.c, host-ui-imgui.cc:
  host-ui.h describes the API, and the other ones implement it.
  - Reverted host-ui-simple.c to just using the pre-transformed vertex
    buffer (no projection matrix).
* Separate `host_render_video` into `host_begin_video` and
  `host_end_video`. The ImGui rendering goes in between.