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

nit: may I suggest we choose a more unique name (e.g. "android_halide_gl_native") now, to minimize confusion if/when we add additional Android JNI tests in the future?