There are still some minor sins in here -- we should really unfuse
attributes from buffers completely -- but this is enough to have vertex
fetch work for all per-vertex attribute scenarios.

Bug: b/124177079

Change-Id: I2a7a1a6f049aa80c1a527e9fa9643bb33701d165
Reviewed-on: https://swiftshader-review.googlesource.com/c/25448Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>