Support for creating nanovg context for OpenGL 2, OpenGL ES 2, OpenGL 3, and OpenGL ES 3.

Now the WidgetView class creates nanovg context based on the configured OpenGL version. Currently all configurations are set explicitly in the moui.gyp file. Both Android and iOS use OpenGL ES 2, all others use OpenGL 3.

Signed-off-by: Olli Wang <olliwang@ollix.com>
Initializes moui view for Mac.

This commit implements the View and NativeView classes for Mac. In addition, a MOOpenGLView class inherited from NSView is implemented to provide OpenGL environment for Mac.

Like iOS, Mac also uses premultiplied alpha for blending. Currently, only OpenGL 2 is supported for Mac.

Signed-off-by: Olli Wang <olliwang@ollix.com>