added 13-stencil example
Merge branch 'master' of https://github.com/dariomanesku/bgfx

Conflicts:
	examples/13-stencil/fs_stencil_color_lightning.sc
	examples/13-stencil/fs_stencil_color_texture.sc
	examples/13-stencil/fs_stencil_texture.sc
	examples/13-stencil/fs_stencil_texture_lightning.sc
	examples/13-stencil/stencil.cpp