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and render to a combined color+alpha bitmap
The strategy is to remove the mpAlphaVDev from OutputDevice,
and have the OutputDevice create 32-bit SalBitmap's
and then have the various backends correctly render to
those 32-bit bitmaps.
However, whenever we extract a BitmapEx from OutputDevice
(e.g. via VirtualDevice) and hand it to the rest of the
system, we split the color and alpha information.
So the rest of the codebase doesn't have to deal with
this yet.
We have some major problems to overcome here:
(*) Cairo uses 32-bit data, even if we ask for 24-bit color with no alpha.
This is because the current version of cairo does not support RGB 24-bit data.
(*) Sometimes we want the bitmap to be initialised with an opaque white background,
sometimes we want it to be transparent.
(*) Sometimes when we extract a bitmap from a device, we want the alpha channel,
sometimes not.
(*) There are lots of left-over bits of previous attempts of this change lying around,
e.g. in skia.
Notes
(*) we allow Bitmap (ie. SalBitmap) to hold color+alpha
(*) The BitmapEx(Bitmap) constructor checks if the incoming
Bitmap is color+alpha, and if so, splits into a color bitmap and an alpha bitmap
(*) Because the underlying image data is stored pre-multipled, if
we write a transparent pixel with color, and then read the pixel value back,
we might lose the color information.
Change-Id: I0fc0042d6e5be5edd99b320892fb00a8eb2842fe
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/173937
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>