app: improve application lifetime handling

[w23]

Currently it's just:

1. app starts with some gl context
2. app exits with basically exit()/abort()

Consider something a bit more detailed:

1. app_init_before_gl() <- have a way to read arguments, do some logic to select gl version, fullscreen mode, preferred desktop, etc etc
2. app_init_after_gl() <- after window and gl created
3. app_destroy() <- function to handle app-specific teardown