Make text rendering optional in pathfinder_canvas

[est31]

Fixes #164
Fixes #208

[est31]

cc @s3bk this is what you wanted I think

[est31]

I've tested @s3bk 's pdf viewer with this patch and it works great.

[s3bk]

thanks!
Do you think it would be cleaner to move the text stuff into canvas/src/text.rs ?

[est31]

@s3bk yeah it's cleaner. I've pushed a commit to move the text stuff into separate files.

[pcwalton]

I generally prefix all Pathfinder features with pf- to avoid conflicts with other crates.

[est31]

re-r? @pcwalton

[pcwalton]

Cool, thanks!