The spec was recently changed to clarify that this should throw an
error.

Once FBOs are allowed, we were running into testcases that deleted the
active FBO, and expected the following ReadPixels to come from the
default framebuffer.

For now it's returning the default UNSUPPORTED on user FBOs.

object-deletion-behaviour.html starts running a bunch more subtets.

This is not a complete implementation yet: It doesn't clear the
contents of the renderbuffer on creation.  However, Gecko's plan to
only clear renderbuffers when the first FBO using them is the
simplest.

This allows many FBO tests to start running as their framebuffers
start coming back as framebuffer complete.

This is part of general GL behavior: when an object is deleted, look
through the currently bound objects and detach the deleted object from
them.  Detaching an object from an FBO causes it to need to be
re-checked for its status.

When the FBO has been unbound, anything that happened to its
attachments (deletion, new binding, reallocation) may have caused it
to change status.

This doesn't change any test results currently, because we're not
checking if the attachments are deleted, the wrong size, or have
appropriate formats.