Correct draw order for frame tree layers
Implement compositor layer occlusion; clean up
auto merge of #758 : eschweic/servo/comp-clean-up, r=metajack

Reorder layer tree construction, implement layer occlusion, and general clean up.
support android build
add android main
add some android port materials into gfx
hybrid window system - glut for android and glfw for others
fix dom bindings for android(and 32bit system) build
fix compile err (internal compiler err on ARM)
update submodules
add native submodules for android
add rust-glut for android
Servo-Android App packaging works