MEDIUM: mux-h2: start to introduce the window size in the offset calculation

MEDIUM: mux-h2: start to introduce the window size in the offset calculation

Instead of incrementing the last_max_ofs by the amount of received bytes,
we now start from the new current offset to which we add the static window
size. The result is exactly the same but it prepares the code to use a
window size combined with an offset instead of just refilling the budget
from what was received.

It was even verified that changing h2_fe_settings_initial_window_size in
the middle of a transfer using gdb does indeed allow the transfer speed
to adapt accordingly.