mesa: add env var to force enable the KHR_no_error ctx flag

V2: typo know -> known
V3: add security check (Suggested by Nicolai)

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>

docs/envvars.html

@@ -46,6 +46,9 @@ sometimes be useful for debugging end-user issues.
 <li>MESA_NO_MMX - if set, disables Intel MMX optimizations
 <li>MESA_NO_3DNOW - if set, disables AMD 3DNow! optimizations
 <li>MESA_NO_SSE - if set, disables Intel SSE optimizations
+<li>MESA_NO_ERROR - if set error checking is disabled as per KHR_no_error.
+   This will result in undefined behaviour for invalid use of the api, but
+   can reduce CPU use for apps that are known to be error free.</li>
 <li>MESA_DEBUG - if set, error messages are printed to stderr.  For example,
    if the application generates a GL_INVALID_ENUM error, a corresponding error
    message indicating where the error occurred, and possibly why, will be