Crisp text rendering when positions are integer values

[janpaul123]

It's currently hard to even manually get good font hinting (barring implementing all of #169..). Some basic fixes to this probably include:

1. Make it so subpixel_x_fract and subpixel_y_fract are 0 when rendering at integer positions.
2. Make it so with our default fonts glyphs are optimally hinted when rendering at integer positions (might also include using integer default values for stuff in TextStyle.
3. Round up widths and heights for layout boxes in TextIns::draw_walk
4. Add a mode where warnings are logged when rendering with non-integer values
5. Add basic "grid rendering" mode where advance_width and initial positions are rounded.

Combined with a debugger #174 this would help a lot.