non composited screens without opengl use nasty rgb paint

[totaam]

Reproduced with openbox as window manager, which doesn't support compositing and therefore no transparency. Since I run it in a VM, I don't have opengl acceleration either.
We end up calling paint_rgb24 and that fails to find a matching cairo format to use for the BGRX pixel data received:

compress:   0.1ms for    4x102  pixels at 1088,108  for wid=21    using     rgb24 with ratio  10.9%  (    1KB to     0KB), sequence   247, client_options={'rgb_format': 'BGRX', 'lz4': 3, 'encoder': 'argb'}, options={'quality': 97, 'speed': 19, 'rgb_formats': ('YUV420P', 'YUV422P', 'YUV444P', 'GBRP', 'BGRX', 'RGBX', 'RGB', 'BGR'), 'lz4': True, 'alpha': False, 'content-type': 'browser', 'window-size': (1094, 1075), 'cuda-device-context': None}
cairo_draw: backing=<cairo.ImageSurface object at 0x7fbd49b5cdb0>, size=(249, 48), render-size=(249, 48), offsets=(0, 0, 0, 0), pointer_overlay=None
cairo._do_paint_rgb(RGB24, True, 47808 <class 'memoryview'>, 0, 0, 249, 48, 249, 48, 996, typedict({'rgb_format': 'BGRA', 'quality': 100, 'has_alpha': True, 'encoder': 'webp', 'encoding': 'webp'})) set_image_surface_data=<built-in function set_image_surface_data>, use pixbuf=False
cannot set image surface data for cairo format 1 and rgb_format BGRA (rgb formats supported: ('RGB', 'RGBX', 'BGR', 'BGRX'))
nasty_rgb_via_png_paint(cairo.Format.RGB24, True, 47808, 0, 0, 249, 48, 996, 'BGRA')

Also, the window 0x18 changed its transparency attribute is Firefox creating a transparent overlay window. Not sure what to do about this one.

[totaam]

Very easily reproducible by running the client with 544dee1 or later:

xpra start :10 --start="xpra example transparent-window"
xpra attach :10 --env=XPRA_ALPHA=0 --no-mmap --opengl=no

The server ends up using webp in BGRA mode:

compress:   2.4ms for  320x320  pixels at    0,0    for wid=1     using      webp with ratio   0.7%  \
    (  400KB to     2KB), sequence     5, \
    client_options={'rgb_format': 'BGRA', 'quality': 100, 'has_alpha': True, 'encoder': 'webp'}, \
    options={'quality': 100, 'speed': 80, 'rgb_formats': ('BGRX', 'RGBX', 'BGR', 'RGB', 'r210', 'BGR565'), \
    'lz4': True, 'alpha': False, 'window-size': (320, 320), 'cuda-device-context': None}
nasty_rgb_via_png_paint(cairo.Format.RGB24, True, 409600, 0, 0, 320, 320, 1280, 'BGRA')

[totaam]

The commit above fixes the problem, but also uncovers another one: #3648