If an Emscripten app is in relative mouse mode and the user presses Escape
(or whatever is appropriate), then the pointer lock is broken by the browser.
This keeps track of those losses, and next time the user presses a mouse
button down on the canvas, if the app is still meant to be in relative mouse
mode, we will attempt to regrab the pointer.
This makes it much more seamless for things like first-person shooters, and
the app doesn't need any manual intervention.
The internal function SDL_EGL_LoadLibrary() did not delete and remove a mostly
uninitialized data structure if loading the library first failed. A later try to
use EGL then skipped initialization and assumed it was previously successful
because the data structure now already existed. This led to at least one crash
in the internal function SDL_EGL_ChooseConfig() because a NULL pointer was
dereferenced to make a call to eglBindAPI().
