This fixes an issue where the player could get stuck to an edge when
being aligned to the pixel grid due to the rounded coordinate being
outside an invalid tile but the truncated position being inside. Also
we should be doing it anyway.
Also changed the game_update() interface to take dt in seconds as a
double (rather than milliseconds as an unsigned integer) as we're now
computing the refresh interval as a double anyway (by querying the
display mode). To my eye (and on my machine), all chunkiness is now
gone; the game feels very smooth.
This fixes an issue where tiles around the right and bottom edges
would not be drawn when vpos was just under a tile boundary. I'm
guessing it wasn't noticable before fixing the frame rate as the buggy
frames were only on-screen for a hilariously short amount of time.
