Generate vertex tangents for OBJ files

[GrantMoyer]

Currently, rendy::mesh::obj::load_from_obj() does not generate vertex tangents when loading OBJ files. This prevents OBJ files from being used with Amethyst's RenderBundle's RenderPbr3D plugin, which needs vertex tangents.

I can implement this, but I want to determine if this is a desired feature, and if so, what tradeoffs are preferred before I start. There are 3 ways I see to implement this:

1. Just generate the tangents all the time

• + Just works
• - Wastes CPU cycles if tangents aren't needed

2. Add a boolean parameter to specify if tangents are desired

• - Breaks API
• - Inflexible; tangents treated as more special than other vertex attributes

3. Make load_from_obj generic over a new trait and implement the trait for Vec<A> where A: AsVertex and tuples of those types

• + Leaves room for future extension
• - Breaks API
• - Adds type complexity

My intuition is that 1 is the best option if OBJ loading time is considered negligible, and 3 is the best option otherwise.

[GrantMoyer]

Related issue: amethyst/amethyst#2104