This may be masking another bug, but at least what happens in
Phant1 is that the mouse does not get redrawn after being moved
programmatically to the fast-forward button when deviceMoved
returns early.
More than one call to OSystem::updateScreen per frame on systems
with vsync ruins performance because the call is blocked until
the next vsync interval.
This also fixes bad rendering performance with the OpenGL backend.
This flag was used in SSCI to read from VRAM instead of from the
back buffer when a mouse interrupt was received in the middle of
a back buffer update. Since ScummVM controls when mouse events
are received via polling, it is not possible to receive a mouse
event in the middle of back buffer updates, so this code is
unnecessary for the engine to work properly.
This also fixes Valgrind warnings about use of uninitialized
memory at the start of the game, caused by not filling the cursor
memory buffers because `_frameNowVisible` was false until the first
frame was rendered.
