High CPU consumption when in different workspace

[claudiocabral]

In OS X CPU consumption goes to 100% if etudes is minimized or in a virtual desktop that's not being displayed.

[drlight-code]

As discussed, it seems that GLFW will return immediately from glfwSwapBuffers if the window is not visible. What we should do here is remember the time after the last buffer swap, and if we return early put the thread to sleep for the difference to 1/60s for a 60Hz framerate, which we also get with the regular vsynced buffer swap.

[drlight-code]

This issue is reported in GLFW: glfw/glfw#680
It affects only Mac OS X and is somehow related to, or can be circumvented by the "App Nap" feature. We will for now not be dealing with this ourselves in etudes. We might suspend the rendering when the window is minimized, however it also occurs if the window is fully obstructed by another window, and I don't see an easy way how to deal with that. We might introduce a global framerate throttling of 90-100Hz or so, however, so I'll keep the issue open for now.