Fix sign error in CoreText font rasterizer
Fix sign error in CoreText font rasterizer

Descent was being reported as a positive value instead of negative. This
caused the background and text alignment to be off dramatically.

Resolves #545

alacritty/alacritty,jwilm/alacritty,alacritty/alacritty,jwilm/alacritty,jwilm/alacritty,jwilm/alacritty