aviobuf: Increase the default SHORT_SEEK_THRESHOLD to 32 KB

aviobuf: Increase the default SHORT_SEEK_THRESHOLD to 32 KB

The previous threshold, 4 KB, maybe was reasonable when it was set
(in 2010), but in today's settings and with typical network speeds
and data sizes, it's pretty small. 32 KB probably is a more reasonable
default now, regardless of input.

This changes the test references for two seek tests.

When using the normal seek function, which boils down to the lseek(2)
function, a seek to an out of bounds position doesn't return an error,
but that condition is only reported when doing the subsequent read
(which returns EOF). When doing more seeks by fast forwarding, the
fact that the seeked to destination is out of bounds is noticed and
reported sooner in these cases.

Signed-off-by: Martin Storsjö <martin@martin.st>