Made RNGLContext.java thread-safe (fixes rare exception when a shader is used in multiple surfaces at the same time)

+ can potentionally explain/fix other witnessed crashes

react-native no longer has a createJSModules function, so no @Override required.

Exposing GLCanvas.setZOrderMediaOverlay as React Native prop. Setting
it to true put the new SurfaceView above the previous. Fixes issue #131

Fixes https://github.com/gre/gl-react-image/issues/1

see https://github.com/ProjectSeptemberInc/gl-react/issues/60

Use with https://github.com/ProjectSeptemberInc/gl-react/commit/0253046f326d4e4d6f088a82b81f2daa955623b9

- format: "base64" || "file"
- type: "png" || "jpeg"/"jpg" || "webm" (android only)
- quality: from 0 to 1 (iOS support only with type=jpg/jpeg)
- filePath: for format=="file" you must provide a file path (you can use react-native-fs to get directory constants)

Attempt to fix #38

See https://github.com/ProjectSeptemberInc/gl-react/issues/38