* Most of the main entry points that can fail with relevant reasons now has a
way of specifying a message to return with it. This message can be displayed
to the user to give more information or context about an error.
* There's not a good accepted terminology for this kind of event, and for
historical reasons 'drawcall' has been the accepted term, even though
that can be quite confusing when a dispatch or a copy is a 'drawcall'.
* This is particularly highlighted by the event browser filters where
$draw() includes draws and dispatches, but $dispatch() only includes
dispatches, it's hard to intuitively understand why $draw() matches all
of these calls.
* As a result we've defined the term 'action' to cover these types of
events in the same way that we defined 'event' in the first place to
mean a single atomic API call.
* This is better than testing by platform/platform version, because we can check
specifically for extensions and avoid running a test we know will fail because
extensions aren't supported.
* This is required because we add MAP_READ_BIT and the spec doesn't allow maps
that are both unsynchronized and reading.
* Also add a test for unsynchronised persistent maps, and update the
GL_Buffer_Updates test to not require a reference image but check each quad
individually for the expected colour.
baldurk