graphics: Remove roundTexel function
graphics: Remove roundTexel function

Actually, at least 704d4cf4640402bd964c77a28938e16646e8dd09,
examples/moire doesn't work well without roundTexel on MacBook Pro.
However, now other parts are fixed (e.g. highp is used), it looks
like roundTexel is no longer needed.

hajimehoshi/ebiten,hajimehoshi/ebiten