tdf#156630 eliminate opaque parts when drawing animated PNG images

Due to the switch from transparency to alpha in commit
81994cb2b8b32453a92bcb011830fcb884f22ff3, flip the background colors
for the VirtualDevices and invert an alpha mask.

On macOS, with Skia/Raster with a Retina display (i.e.  2.0 window
scale), the alpha mask gets upscaled. Also, when Skia is enabled,
the alpha mask gets inverted in the first export to PDF after
launching the application. These two bugs appear to be caused by
asynchronous rendering of the returned bitmap. So, we force a copy
of the alpha mask in case it changes before the bitmap is actually
drawn.

Lastly, respect system animation settings when determining if the image
should be animated.

Change-Id: I8144691a6c99bf8361b301b88d22172991463f26
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/155429
Tested-by: Jenkins
Reviewed-by: Patrick Luby <plubius@neooffice.org>

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