crash with gtk3 cairo backend

[Antoine Martin]

python3 /usr/bin/xpra attach tcp://127.0.0.1:10000 --opengl=no -d all

Shows:

2018-06-24 23:47:24,690 cairo._do_paint_rgb(ARGB32, True, 630736 \
    <class 'xpra.net.mmap_pipe.c_char_Array_630736'>,15,0,484,316,1996, \
    {b'rgb_format': 'BGRX', 'encoding': 'mmap'}) \
    set_image_surface_data=<built-in function set_image_surface_data>, use pixbuf=False
2018-06-24 23:47:24,690 source image surface: (cairo.Format.ARGB32, 484, 316, 1936, cairo.Content.COLOR_ALPHA)
2018-06-24 23:47:24,691 cairo_paint_surface(<function CairoBackingBase.cairo_paint_surface.<locals>.set_source_surface at 0x7fc0b771a048>, \
    <cairo.ImageSurface object at 0x7fc09ed645f0>, 15, 0, 484, 316, \
    {b'rgb_format': 'BGRX', 'encoding': 'mmap'}) backing=<cairo.ImageSurface object at 0x7fc0b421f910>, paint box line width=0
double free or corruption (out)
Aborted (core dumped)

The bug does not occur with opengl on, or with mmap off.
Converting the mmap array to bytes does not help.

[Antoine Martin]

Not just with mmap, also with plain rgb.

Memory overflow fixed in r19707.

Related improvements in:

• r19705: more consistent cairo context state
• r19706: simplify surface setup code
• r19708: better / faster pixel copying
• r19709: release the GIL (it would be hard to measure if this is actually beneficial as this can't easily be turned into a runtime switch)