These fixes allow dEQP 3.1 KHR-GLES31.core.vertex_attrib_binding.basic-input*
test cases to pass.

This fixes a bug in instanced drawing with transform feedback buffers
where we were overwriting the initial transform feedback output as new
instances were drawn. To fix this, the vertices in each draw are passed
into a uniform value and the output is offset based on the current instance
and the number of vertices in the draw.

It also fixes a validation error where transform feedback descriptor
was sometimes setting a buffer range of 0. This is not allowed, so in
the 0 case we pull the actual buffer size from the bufferHelper object.

Bug: angleproject:4236
Change-Id: Ib72898a0e6caab96c446c1e996a6124d2c001193
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1979600
Commit-Queue: Tobin Ehlis <tobine@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Tobin Ehlis <tobine@google.com>