Add use of the KHR_no_error extension

Supported window toolkits:
* [GLFW](http://www.glfw.org/docs/latest/window_guide.html) using the `GLFW_CONTEXT_NO_ERROR` flag

Unsupported window toolkits:
* Qt

First tests using the Mesa implementation indicates small but noticable performance improvements: https://www.phoronix.com/scan.php?page=news_item&px=Mesa-17.2-Trying-NO-ERROR
I suggest we use this exension (if available) in Release mode as counterpart to our extended debug facilities in Debug mode.

The issue names no files, tests, or entry points. Start by locating context creation for GLFW and Qt, then compare Release and Debug configuration handling. Done means using KHR_no_error where available, retaining the documented Qt limitation, and confirming the GLFW path uses GLFW_CONTEXT_NO_ERROR.

Written by the indexing model from the issue text.