render: eliminate grotesque stupidity

We were doing an entire copy of the framebuffer every fucking
render for no good reason whatsoever. Found this while working
on #83, but let's get it in master immediately. JFC. I suck.

Before (delay factor of 0.5):

[schwarzgerat](0) $ time ./notcurses-demo -d.5
Term: xterm with direct-color indexing
1737 renders, 24.8s total (0.00131s min, 0.362s max, 0.014s avg 70.1 fps)
230217.182KB total (12.696KB min, 223.310KB max, 132.54KB avg)
Emits/elides: def 1074878/126257 fg 6615070/3023957 bg 5519815/3130192
 Elide rates: 10.51% 31.37% 36.19%

real	0m47.653s
user	0m8.977s
sys	0m1.414s
[schwarzgerat](0) $

After (same delay factor of 0.5)

[schwarzgerat](0) $ time ./notcurses-demo -d.5
Term: xterm with direct-color indexing
1741 renders, 23.2s total (0.00125s min, 0.351s max, 0.013s avg 75.0 fps)
231551.089KB total (12.696KB min, 223.310KB max, 133.00KB avg)
Emits/elides: def 1065445/126257 fg 6655295/3006679 bg 5566155/3115184
 Elide rates: 10.59% 31.12% 35.88%

real	0m46.140s
user	0m8.932s
sys	0m1.350s
[schwarzgerat](0) $

Now *there's* the loss we saw during #139 work, thinking we'd get it
back in #143. Good. Huzzah! But also fuck!

src/lib/notcurses.c

@@ -381,6 +381,9 @@ int notcurses_resize(notcurses* n, int* rows, int* cols){
   if(update_term_dimensions(n, rows, cols)){
     return -1;
   }
+  if(*rows == oldrows && *cols == oldcols){
+    return 0; // no change mang
+  }
   ncplane* p = n->stdscr;
   cell* preserved = p->fb;
   size_t fbsize = sizeof(*preserved) * (*rows * *cols);