Windows: Don't lose the existing current GL context in SDL_CreateWindow().

Fixes Bugzilla #1395.
Fixed bug 1569 - Android volume keys not honoured inside SDL

Joseba García Echebarria

The current version of SDL HG in Android doesn't do anything when volume keys are pressed.
The change makes SDL ignore volume keys so that they're handled by the OS and the sound volume can be changed within an app.
Documented why we have global and thread-local variables for the GL context.
Move the fix for Bugzilla #1395 into WIN_GL_SetupWindow() directly.

It's a cleaner solution.

--HG--
extra : rebase_source : efd29bea6e39b6aed73f78a0f119b3b4c2bbd498
More work on bug 1999, added missing CFBundleExecutable key to Info.plist