Fixed binding offsets for uniform and transform feedback buffers

Fixed binding offsets for uniform and transform feedback buffers

There was some confusion between buffer mapping and size and offset
buffer binding arguments, which are distinct, but were represented
by the same Buffer class members. Added OffsetBindingPointer to solve
the issue and removed setOffset/setSize from the Buffer class, which
should not have existed in the first place (only the mapRange/unmap
functions should be allowed to modify these values, for now).

Change-Id: Iacecd17cfb90d0a229d9edf62a463c8acf31f07a
Reviewed-on: https://swiftshader-review.googlesource.com/4590
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>