Calculating extents using freetype-go

[BurntSushi]

The freetype-go package API doesn't yet support computing text extents without actually rendering the text to the image. Currently, the workaround is to calculate a "maximum" bounding box using the number of pixels in a single em unit. While this prevents text from ever being cut off, it tends to dramatically overestimate when fonts aren't monospace (since many characters have a width or height less than one em unit). Thus, the image is then cropped using the (x, y) offset returned from (*Context).DrawString.

Ideally, we should be able to calculate extents before rendering, therefore saving wasteful allocation. (For my use cases, the wasted allocation isn't too big of a deal.)

For more details, see Issue 5 on the freetype-go issue tracker.