Two bugs were affecting the following scenario:

- Create program with xfb and no default uniforms
- Draw without xfb
- Begin xfb
- Draw with xfb
- End xfb
- map and validate

The first bug was that the descriptor set that contains the xfb buffers
and default uniforms was never allocated for the first draw call
resulting in VVL errors and crashes.

The second bug was that "Begin xfb" was not setting the xfb buffers
dirty bit, which meant that after the first draw call bound dummy
buffers to the descriptor set, they would not get updated.  This was
simultaneously fixed in 5d3a4ca4, but the added test nevertheless tests
this.

Bug: angleproject:4660
Change-Id: I16eee315e39ec5a657481e54ecd0491581e886aa
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2376172Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>