baseVertex is a signed integer that's allowed have a negative value.
In order to support it, the offset was moved from being applied
on the index buffer memory pointer directly to being applied
to the indices from the index buffer inside the Vertex routine.

Fixes all issues in the following subcategory using SwANGLE:
dEQP-GLES31.functional.draw_indirect.*

Bug: b/144353667
Change-Id: Ia70c258eab25c274553c4955dc2336a42d43f005
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38148
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>