Renderer side code to lock/unlock transform feedback data

Renderer side code to lock/unlock transform feedback data

Added variables and function calls to lock/unlock transform
feedback data used for reading the selected transform
feedback varying variables out of the vertex shader.

Also fixed uniform buffer unlocking potential race condition.

Change-Id: I98790fc36fdf8674506d924b2f21b3e68892811a
Reviewed-on: https://swiftshader-review.googlesource.com/5060
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>