Handle "OpenGL context lost" event on Android

- added a new runtime function `halide_opengl_context_lost` that resets all context-specific state held by Halide
- update HelloAndroidGL example to handle context lost events correctly
- extend example app to optionally render directly to the current framebuffer instead of going through a texture
- bring README file up-to-date
I do like `do_not_bind_output` .

I'd like to rename the function in a separate PR, however - not only because it's unrelated to the changes in this pull request, but also since we probably have to change the function anyway with the upcoming GPU runtime reorganization.