Add draw_fps helper function

[juliohq]

This PR adds draw_fps and draw_frame_time functions for drawing the result of get_fps and get_frame_time, respectively.

The rustaceanmark example was updated to use the newly added draw_fps function.

[not-fl3]

Thanks for PR!
I was thinking about draw_fps, and I think that in a shape of draw_fps(0., 16., 32., WHITE); it doesn't really provide enough value to justify an existance of a separate function. And I don't really think anyone would ever use draw_frame_time, and if they will need it - I would say its too niche of the use case for a special function.

However, draw_fps() with no arguments might be usefull for quick tests, what do you think about this?
I would add a doc-comment that draw_fps() is just a shortcut for draw_text(..) and for customization draw_text is preferable way to do it.

[juliohq]

Yes, I think it makes sense to remove draw_frame_time in this case and add the doc-comment you mentioned to draw_fps.

[not-fl3]

I would rather have draw_fps() as a no-argument function. If custom format, position, font size of anything is required - its not that much of a problem to call draw_text I belive.

[juliohq]

I would rather have draw_fps() as a no-argument function.

Sure, it should work better like that I think.

[not-fl3]

Perfect, thanks for PR!