Update FlextGL.

See flextGL commit 5d27886203242e4bc22d5390efcd55cd52eb3e11. This fixes an
issue with glGetStringi/glGetString mechanism failing on some platforms with
certain OpenGL versions.
Bump version 0.3.0.0 --> 0.3.0.1
Refine test program sdl2 version requirements (1.* --> 1.2.*)

They appear not to work with sdl2 1.3.*
Update FlextGL.
Add the files required to generate FlextGL.hs.

Previously I only had these on my own computer, out of public sight. This is no
good so I put them in the repository.
Abstract over MonadIO