Transparency clickthrough

This pull request will also needs https://github.com/rogerwang/chromium.src/pull/30

to enable Transparency clickthrough (currently only supported on OSX / WIN)
you must run with these args "--disable-gpu --force-cpu-draw"

I've added global variable g_force_cpu_draw which by default is false, only set to true if the command arg --force-cpu-draw specified

this pull request also has fixes for:
regression https://github.com/rogerwang/node-webkit/issues/1021
and https://github.com/rogerwang/node-webkit/issues/2784