libvterm / pangoterm performance improvements

I recently made a couple of small but drastic improvements to libvterm and pangoterm's performance, when scrolling long output. Using my not-so-scientific test of taking a ~500KB text file and running:

$ time cat file

This technically measures the amount of time it takes cat to write the data into the TTY, but since the buffer is a fixed 4KiB in size, it also measures the time that pangoterm takes to read all but the final 4KiB of the file, which is under 1% of the file. I managed to obtain the following timings. Before I started:

real    0m4.206s
user    0m0.000s
sys     0m0.036s

By optimising libvterm's moverect buffer operation with memmove() (revision -r511):

real    0m2.035s
user    0m0.000s
sys     0m0.048s

With pangoterm deferred updates, that delay re-rendering of the screen until all the PTY data has been read, or every 20 msec (revision -r482):

real    0m0.358s
user    0m0.004s
sys     0m0.016s

It's now well over 10 times faster than it used to be. Ohh.. and it beats xterm. By quite a bit.

real    0m2.294s
user    0m0.000s
sys     0m0.036s