Fix render issues on macOS (#845)
Fix render issues on macOS (#845)

Parts of neighboring glyphs in the atlas were being rendered
incorrectly. The issue is resolved by aligning cells to the pixel grid.

This behavior was achieved previously by first applying integer
truncation before casting to a float.

Fixes #844.
alacritty/alacritty,jwilm/alacritty,jwilm/alacritty,alacritty/alacritty,jwilm/alacritty,jwilm/alacritty