D3D9 has a special limitation on varying packing, where each
variable takes up a full register width, and cannot share space
with other packed varyings.

A bug was counting registers incorrectly on D3D9. Fix this by
introducing a new limitation exposed to the ANGLE front-end via
the gl::Limitations structure. Now varying packing will fail
correctly in the ANGLE linking front-end with a more descriptive
error message, as such:

"Could not pack varying blah"
"Note: Additional non-conformant packing restrictions are enforced on D3D9."

Also change the packing so that input built-in variables are
counted towards varying limits (e.g. gl_PointSize), except for
gl_Position. On D3D9 we don't pack gl_PointSize, since it is
used in a special extra PSIZE register.

Also update some tests to be more robust.

Bug: chromium:804799
Change-Id: I9027266a8b66a28626f038f259bff42ebf09dcd2
Reviewed-on: https://chromium-review.googlesource.com/889898
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>