Support SDL_ActiveEvent

Support SDL_ActiveEvent 

SDL tracks three kinds of application state: SDL_APPACTIVE, SDL_APPINPUTFOCUS, 
and SDL_APPMOUSEFOCUS. The first corresponds to whether the window is 
minimized, the second to keyboard focus, and the third to whether the mouse is 
over the window. For NaCl, I believe the best analogues are 
PP_INPUTEVENT_TYPE_MOUSEENTER and PP_INPUTEVENT_TYPE_MOUSELEAVE for 
SDL_APPMOUSEFOCUS, keyboard focus for SDL_APPINPUTFOCUS, and page visibility 
for SDL_APPACTIVE. 

SDL_APPMOUSEFOCUS can be supported easily with SDL_NACL_PushEvent. For the 
other two, I believe we need new entry points, and possible even JS postMessage 
involvement. Page visibility is exposed in pepper 18 via 
pp::Instance::DidChangeView(View) and pp::View::IsPageVisible(), but that's not 
on stable yet. Focus should be pp::Instance::DidChangeFocus(bool), but that 
appears to be updated only when WebKit-internal focus changes. It doesn't notice 
when you switch tabs or windows, unlike DOM focus and blur events. Unclear if 
this is intentional or an oversight in the implementation. 

The main thing this buys is that SDL calls SDL_ResetKeyboard() when input focus 
is lost. This means it, e.g., generates a fake key-up event for Ctrl when 
switching tabs with Ctrl-Tab, otherwise the module keeps thinking Ctrl is held 
down. Some ports may also useful things with the others, such as pausing when 
SDL_APPACTIVE is lost. 

Implementation-wise, this is a little invasive as it rewrites the internal 
event queue to use SDL_Event as the intermediate structure as that seemed 
cleanest.

Landing a change by David Benjamin, reviewed at
https://chromiumcodereview.appspot.com/9860021/
Committed: https://code.google.com/p/naclports/source/detail?r=568

[robertm, 2012-04-03 13:47:24]

LGTM