add additional 0-1 alpha argument to DrawTransformedBitmap()

This allows the VCL backends the apply the extra alpha transformation
as it sees fit, rather than it being done manually elsewhere (and
even if the backend doesn't implement it, at least do it in one
place in the function).
With the document from tdf#136223, going from slide 2 to slide 3,
this easily saves 10-30% of CPU cycles. As an additional bonus,
using AlphaMask::BlendWith() rather than AlphaMask::Replace()
makes edges of shapes noticeably more smooth.

Change-Id: I036dc9b887d6def0c7cdad3982becabdc7cd5206
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/111247
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/vcl/inc/salgdiimpl.hxx b/vcl/inc/salgdiimpl.hxx
index eb1508dcf7d6..8b310c586e1a 100644
--- a/vcl/inc/salgdiimpl.hxx
+++ b/vcl/inc/salgdiimpl.hxx
@@ -194,7 +194,8 @@ public:
                 const basegfx::B2DPoint& rX,
                 const basegfx::B2DPoint& rY,
                 const SalBitmap& rSourceBitmap,
-                const SalBitmap* pAlphaBitmap) = 0;
+                const SalBitmap* pAlphaBitmap,
+                double fAlpha) = 0;
 
     /// Used e.g. by canvas to know whether to cache the drawing.
     virtual bool hasFastDrawTransformedBitmap() const = 0;