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 agree that the name isn't optimal... what about `halide_opengl_render_to_screen`? @abestephensg : any opinions?

Good point regarding namespace/extern "C", I moved the two functions to extern "C" block at the bottom of the file.