This adds SDL_SetWindowKeyboardGrab(), SDL_GetWindowKeyboardGrab(),
SDL_SetWindowMouseGrab(), SDL_GetWindowMouseGrab(), and new
SDL_WINDOW_KEYBOARD_GRABBED flag. It also updates the test harness to exercise
this functionality and makes a minor fix to X11 that I missed in
https://hg.libsdl.org/SDL/rev/02a2d609369b
To fit in with this new support, SDL_WINDOW_INPUT_CAPTURE has been renamed to
SDL_WINDOW_MOUSE_CAPTURE with the old name remaining as an alias for backwards
compatibility with older code.
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().
