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
outputclient -> output_client

Or something else, if you decide to rename.