bug#30045: X protocol error and a test case

[Vivek Dasmohapatra]

This triggers the crash:

  (defun trigger-font-bork ()
    (interactive)
    (when window-system
      (let ((fontset (face-attribute 'default :fontset))
            (unisyms "Noto Color Emoji"))
        (set-fontset-font fontset '(#x1f900 . #x1f9ff) unisyms))
      (message "No-Boom: %c" #x2615 )
      (message "Boom   : %c" #x1f936)))

Changing unisyms to "Noto Emoji" stops the crash.

Getting out the printf() shotgun shows the crash is triggered around here,
in xftfont_open:

  if (spacing != FC_PROPORTIONAL
#ifdef FC_DUAL
      && spacing != FC_DUAL
#endif  /* FC_DUAL */
      )
    {
       // ...
    }
  else
    {
       // ...
    }
  unblock_input (); // <-- Boom today.

But that's just where the bug catches up with us - fiddling with the
block/unblock input calls can move the crash around a bit.

If we break out xtrace, the bad request can be found:

002:<:15b2: 12: RENDER-Request(139,17):
    CreateGlyphSet gsid=0x036002e1
                   format=0x00000023
002:<:15b3:17436: RENDER-Request(139,20):
    AddGlyphs glyphset=0x036002e1
              glyphids=0x00000441;
              glyphs={width=136 height=128 x=0 y=101 xOff=136 yOff=0};
              data=0x00,0x00,0x00, … } ← 17408 bytes

002:>:15b3:Error 16=Length: major=139, minor=20, bad=56623841

If I swap out the "Noto Color Emoji" font and request "Noto Emoji"
to avoid the crash, we still see AddGlyph requests, but they're much
smaller:

  #reqs size
      2 704
      2 600
      3 588
      3 556
      6 512
      5 508
      2 476
     15 468
      5 460
     16 428
     31 424
      5 412
      5 392
     40 388
     11 380
      3 364
     44 352
     38 348
      6 340
      5 336
     53 316
     11 308
      1 292
      4 288
      6 284
     62 280
     10 268
     55 252
      6 248
     14 244
      3 228
     23 224
      8 220
      7 208
      3 196
     26 188
      4 172
      5 168
      3 160
      2 156
      7 148
      4 140
      5 136
      1 128
      9 124
      1 112
      7 108
      7 100
      3 92
      3 88
      1 84
      5 76
      5 64
      5 60
      2 52
      2 48
      1 40
      6 28

So... either this request is flat-out too large (a bug? there aren't that 
many glyphs in N-C-E) and it needs to be chunked or we need to be telling
X we're going to be making bigger requests somehow.