APPLE_texture_format_BGRA8888 support fix, tested on Apple iPad mini 2.
iOS: added stencil buffer to context fbo. ( fixed stencil example )

Depth/stencil in this fbo ( and other contexts ) should depend on some user settable flag ( flag from bgfx::reset ?)
iOS
- set window size from screen size
- touch input ( WIP: it is not too responsive )
- file path set for resource loading