(Second attempt at landing this)

Image formats and graphics APIs use alpha, not transparency,
so change our internal formats and data structures to work directly
with alpha, so we don't need to modify data before we push it to
graphics APIs.

Add a couple of new Color constants to make the intention
of the vcl code clearer.

Notes
(*) On macOS, tweaking the logic in CreateWithSalBitmapAndMask
to more accurately reflect the requirements of the
CGImageCreateWithMask function seems to fix some
tests.

(*) The vcl code does not properly support gradients
with transparency. So the previous code was wrong, and this
change is going to result in slightly different wrongness.

Change-Id: I9e21c2e98d88ecfdc5f75db13bd1ffff7c38db98
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/114168
Tested-by: Jenkins
Reviewed-by: Patrick Luby <plubius@neooffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

to ease the reading of code related to an upcoming patch to convert
transparency to alpha, since there is already a GetAlpha in Color.

Change-Id: I1af0f8f6dd94acfe4673c8556c7aff6c20da3f7a
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/145209
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

Change-Id: I3b4226a9d089ec9aedab95d96e50a068f57a76c7
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/123991
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

Change-Id: Icb91c4e8aae3f6f25515ca75a3cc230d9238dd4a
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/115370
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>

Change-Id: Iee2e42f3881512d2e779d16505c50f76731e398a
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/115369
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>