method as in /bin/sh.
We still do technically undefined things in the signal handler, but it
is safe in practice to access state that is protected by INTOFF/INTON.
In a recent commit, I sprinkled VGLMouseFrozen++/-- operations in
places that need INTOFF/INTON. This prevented clobbering of pixels
under the mouse, but left mouse signals deferred for too long. It is
necessary to call the signal handler when the count goes to 0. Old
versions did this in the unfreeze function, but didn't block actual
signals, so the signal handler raced itself. The sprinkled operations
reduced the races, but when then worked to block a race they left
signals deferred for too long.
Use INTOFF/INTON to fix complete loss of mouse signals while reading
the mouse status. Clobbering of the state was prevented by SIG_IGN'ing
mouse signals, but that has a high overhead and broke more than it
fixed by losing mouse signals completely. sigprocmask() works to block
signals without losing them completely, but its overhead is also too
high.
libvgl's mouse signal handling is often worse than none. Applications
can't block waiting for a mouse or keyboard or other event, but have
to busy-wait. The SIG_IGN's lost about half of all mouse events while
busy-waiting for mouse events.
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