Transform feedback varyings gather operation

Transform feedback varyings gather operation

A few small things were missing to get the transform
feedback varyings gather operation to work properly:
- A basic implementation of
  Program::gatherTransformFeedbackLinkedVaryings()
  has been made as a first step.
- transformFeedbackBufferMode is now initialized in
  the constructor
- transformFeedbackLinkedVaryings are now properly
  reset

Also:
- Removed useless DirectX semantic information from
  the LinkedVarying class
- ++it is more efficient than it++ in a loop, because
  it++ create a temporary object to return the
  original state of the iterator, so I made the
  changes where applicable in Program.cpp.

Change-Id: I78513f185ef5ef1a17448606b5c598c22d0d217e
Reviewed-on: https://swiftshader-review.googlesource.com/3621
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>