Extension support: maybe not a binary yes/no

Hi,

 On the discussion on Github on dual source blending,
https://github.com/gpuweb/gpuweb/issues/391, it was brought up that some
hardware declared support but it was buggy on some hardware. This led me to
a more general thinking on extensions. There are other cases where a driver
advertises an extension but it does not work correctly and the only way to
know is to either run the test oneself or examine the GPU vendor and driver
string (indeed SKIA has some of this). The thought is that a browser could
provide information if it considers an extension, though advertised by the
driver, it may not work always. So the extension query mechanism would have
three different possible results: "yes", "no" or "yes, but the browser is
suspicious of the driver's support". Given that there are so major browsers
now (and even fewer web-engines), if a browser says the 3rd option, it can
prod IHV's to fix the issue, since small ISV's have much smaller impact
(and will need to implement work-arounds as well).

Comments?

 -Kevin