Expose safe interface to OpenGL timer queries

[nicokoch]

This module provides utilities to profile GPU execution times in a new module glium::timer.

This example shows how to use the new functionalities (from the docs):

// drawing a frame
use glium::timer;
let mut target = display.draw();
let handle = timer::measure(&display, || {
    target.clear_color(0.0, 0.0, 0.0, 0.0);
    target.draw(&vertex_buffer, &index_buffer, &program, &uniforms, &Default::default()).unwrap();
    target.finish().unwrap();
}).unwrap();
println!("GPU needed {:?} to draw", handle.get_measurement().interval());

I had this code lying around from ancient times and ported it to glium.
It makes use of GL_arb_timer_query

[nicokoch]

I'm not really good with the closure traits. Is FnOnce the correct choice?

[nicokoch]

I see something similar has already been implemented in draw_parameters, so maybe this won't be very useful.

How ever, in l. 114 is says FIXME: handle Timestamp separately, so maybe there's hope.

[tomaka]

Isn't that what's already implemented in debug.rs? https://github.com/tomaka/glium/blob/master/src/debug.rs#L119

[nicokoch]

Oh you're right.

What do you think about the api in general?
I know other query objects are currently handled in draw parameters, but my design would allow profiling things like compute shaders (or other non-draw related GL functionality) too.

[tomaka]

Your design works for timestamp queries, but not for other kind of queries because the user can only activate one query of each type simultaneously.