Initial work on infrastructure for GL-hybrid rendering setup.

Conflicts:
	src/Makefile.am
Configure OpenGL on start-up.

Basically a manual partial cherry-pick of
25ac16597e17d01b143c097f768ad3196b06ad8b.
Fix broken errx() format.
Move GL near clipping plane to 0.
Implement (untested) functions to convert canvas to GL texture.
Use GLU to scale canvases onto each other.
Add support for fixed-function pipeline to shader DSL.
Implement system to marshal data to OpenGL from worker threads.
Appropriately maintain vertex array activation states.
Integrate GL marshalling into main.
Add predraw() method to game_state.
Parchment now drawn via OpenGL.
Lay groundwork for porting terrabuff to OpenGL.
Initial port of terrabuff to OpenGL.

Still some weird stuff going on in pixel column zero, and a hole if the camera
is pointed straight down.
Add X shift compensation to GL-based terrabuff rendering.
Make X offsets chaotic in GL-based terrabuff rendering.
Fix hole in terrabuff when looking directly down.
Fix off-by-one error in interpolation boundaries.
Add deactivate step to GLM slab processing.

Fixes numerous races involved in how terrabuff tried to clean up after
rendering.
Re-add drawing_method support to turtle.