Add halide_opengl_context_lost() function

This discards all state associated with the old OpenGL context and forces them
to be re-initialized when the next Halide filter is run.
Update OpenGL documentation
Use render-to-framebuffer in Android example
Merge branch 'master' into glsl_android
Don't put extern "C" functions into a namespace

since the names actually aren't in the namespace...
fix typo
Merge pull request #479 from halide/glsl_android

Handle "OpenGL context lost" event on Android