revert BackendCapabilities::mbSupportsBitmap32 for Skia

It appears that there are still some paths that do not expect
bitmaps to be truly 32bit, so better revert to the old safe (and
poor, complicated and inefficient) way of pretty much ignoring
the alpha channel in SkiaSalBitmap, and let BitmapEx handle
it by using an extra alpha bitmap.

Change-Id: I4318c05f4ceafc5de48e19eeae5efe2abed2ec69

vcl/inc/unx/salinst.h

@@ -81,8 +81,6 @@ public:
 
     virtual void                AfterAppInit() override;
 
-    std::shared_ptr<vcl::BackendCapabilities> GetBackendCapabilities() override;
-
     // dtrans implementation
     virtual css::uno::Reference< css::uno::XInterface >
         CreateClipboard( const css::uno::Sequence< css::uno::Any >& i_rArguments ) override;

vcl/inc/win/salinst.h

@@ -73,7 +73,6 @@ public:
     virtual void                AddToRecentDocumentList(const OUString& rFileUrl, const OUString& rMimeType, const OUString& rDocumentService) override;
 
     virtual OUString            getOSVersion() override;
-    virtual std::shared_ptr<vcl::BackendCapabilities> GetBackendCapabilities() override;
 
     static int WorkaroundExceptionHandlingInUSER32Lib(int nExcept, LPEXCEPTION_POINTERS pExceptionInfo);
 };

vcl/skia/salbmp.cxx

@@ -80,8 +80,16 @@ bool SkiaSalBitmap::Create(const Size& rSize, sal_uInt16 nBitCount, const Bitmap
     }
     if (colorType != kUnknown_SkColorType)
     {
+        // TODO
+        // As long as vcl::BackendCapabilities::mbSupportsBitmap32 is not set, we must use
+        // unpremultiplied alpha. This is because without mbSupportsBitmap32 set VCL uses
+        // an extra bitmap for the alpha channel and then merges the channels together
+        // into colors. kPremul_SkAlphaType would provide better performance, but
+        // without mbSupportsBitmap32 BitmapReadAccess::ImplSetAccessPointers() would use
+        // functions that merely read RGB without A, so the premultiplied values would
+        // not be converted back to unpremultiplied values.
         if (!mBitmap.tryAllocPixels(
-                SkImageInfo::Make(rSize.Width(), rSize.Height(), colorType, kPremul_SkAlphaType)))
+                SkImageInfo::Make(rSize.Width(), rSize.Height(), colorType, kUnpremul_SkAlphaType)))
         {
             return false;
         }

vcl/unx/generic/app/salinst.cxx

@@ -220,14 +220,4 @@ std::unique_ptr<GenPspGraphics> X11SalInstance::CreatePrintGraphics()
     return std::make_unique<GenPspGraphics>();
 }
 
-std::shared_ptr<vcl::BackendCapabilities> X11SalInstance::GetBackendCapabilities()
-{
-    auto pBackendCapabilities = SalInstance::GetBackendCapabilities();
-#if HAVE_FEATURE_SKIA
-    if( SkiaHelper::isVCLSkiaEnabled())
-        pBackendCapabilities->mbSupportsBitmap32 = true;
-#endif
-    return pBackendCapabilities;
-}
-
 /* vim:set shiftwidth=4 softtabstop=4 expandtab: */

vcl/win/app/salinst.cxx

@@ -1067,14 +1067,4 @@ OUString WinSalInstance::getOSVersion()
     return aVer.makeStringAndClear();
 }
 
-std::shared_ptr<vcl::BackendCapabilities> WinSalInstance::GetBackendCapabilities()
-{
-    auto pBackendCapabilities = SalInstance::GetBackendCapabilities();
-#if HAVE_FEATURE_SKIA
-    if( SkiaHelper::isVCLSkiaEnabled())
-        pBackendCapabilities->mbSupportsBitmap32 = true;
-#endif
-    return pBackendCapabilities;
-}
-
 /* vim:set shiftwidth=4 softtabstop=4 expandtab: */