Xwayland crash in glamor/mesa with very large windows

When creating a very large window (width = 32767), Xwayland crashes when using glamor/GBM backend:

(EE) 
(EE) Backtrace:
(EE) 0: Xwayland (OsSigHandler+0x29)
(EE) 1: libpthread.so.0 (funlockfile+0x50)
(EE) 2: libc.so.6 (__memmove_avx_unaligned_erms+0x215)
(EE) 3: dri/i965_dri.so (_mesa_format_convert+0xab3)
(EE) 4: dri/i965_dri.so (_mesa_texstore+0x205)
(EE) 5: dri/i965_dri.so (store_texsubimage+0x28c)
(EE) 6: dri/i965_dri.so (intel_upload_tex+0x13b)
(EE) 7: dri/i965_dri.so (texture_sub_image+0x134)
(EE) 8: dri/i965_dri.so (texsubimage_err+0x150)
(EE) 9: dri/i965_dri.so (_mesa_TexSubImage2D+0x48)
(EE) 10: Xwayland (glamor_upload_boxes+0x246)
(EE) 11: Xwayland (glamor_copy+0x4d1)
(EE) 12: Xwayland (miCopyRegion+0x96)
(EE) 13: Xwayland (miDoCopy+0x43c)
(EE) 14: Xwayland (glamor_copy_area+0x24)
(EE) 15: Xwayland (damageCopyArea+0xba)
(EE) 16: Xwayland (compCopyWindow+0x31c)
(EE) 17: Xwayland (damageCopyWindow+0xd3)
(EE) 18: Xwayland (miResizeWindow+0x7b7)
(EE) 19: Xwayland (compResizeWindow+0x3a)
(EE) 20: Xwayland (ConfigureWindow+0xa96)
(EE) 21: Xwayland (ProcConfigureWindow+0x7d)
(EE) 22: Xwayland (Dispatch+0x320)
(EE) 23: Xwayland (dix_main+0x366)
(EE) 24: libc.so.6 (__libc_start_main+0xf3)
(EE) 25: Xwayland (_start+0x2e)
(EE) 
Fatal server error:
(EE) Caught signal 11 (Segmentation fault). Server aborting
(EE) 

Prior to the crash, the logs show:

XXX fail to create fbo.

While this may look like a glamor/Mesa issue, this is actually an Xwayland bug.