OpenGL on Android

This PR adds Android support to the GLSL backend. Main changes:

* Changes to GLSL codegen and backend to handle OpenGL ES correctly
* New HelloAndroidGL app which demonstrates how to use Halide for rendering to textures
* Cosmetic changes to HelloAndroid app

In some cases you may want to just have the filter assume that there is already a valid context set; in that case, is it safe to simply override halide_opengl_create_context() to return eglGetCurrentContext()?