BPicture: add missing WriteSetBlendingMode method.

Actual blending mode writing is still disabled for now. According to
previous commit messages it cause problems with BView::Begin/EndLayer
API.

Change-Id: Ib3c58b05a949e556e9f14a57822b20cd259e12d4
Reviewed-on: https://review.haiku-os.org/c/haiku/+/8559
Haiku-Format: Haiku-format Bot <[email protected]>
Reviewed-by: Adrien Destugues <[email protected]>
Tested-by: Commit checker robot <[email protected]>

headers/private/interface/PictureDataWriter.h

@@ -42,6 +42,8 @@ public:
 									const join_mode& join,
 									const float& miterLimit);
 			status_t			WriteSetFillRule(int32 fillRule);
+			status_t			WriteSetBlendingMode(source_alpha srcAlpha,
+									alpha_function alphaFunc);
 			status_t			WriteSetScale(const float& scale);
 			status_t			WriteSetTransform(BAffineTransform transform);
 			status_t			WriteTranslateBy(double x, double y);

src/kits/interface/PictureDataWriter.cpp

@@ -171,6 +171,22 @@ PictureDataWriter::WriteSetFillRule(int32 fillRule)
 }
 
 
+status_t
+PictureDataWriter::WriteSetBlendingMode(source_alpha srcAlpha, alpha_function alphaFunc)
+{
+	try {
+		BeginOp(B_PIC_SET_BLENDING_MODE);
+		Write<int16>(srcAlpha);
+		Write<int16>(alphaFunc);
+		EndOp();
+	} catch (status_t& status) {
+		return status;
+	}
+
+	return B_OK;
+}
+
+
 status_t
 PictureDataWriter::WriteSetScale(const float& scale)
 {

src/servers/app/ServerWindow.cpp

@@ -4099,23 +4099,22 @@ ServerWindow::_DispatchPictureMessage(int32 code, BPrivate::LinkReceiver& link)
 			break;
 		}
 
-/*
+// TODO: disabled without explanation in hrev19264. investigate problem and enable
+#if 0
 		case AS_VIEW_SET_BLENDING_MODE:
 		{
 			ViewBlendingModeInfo info;
 			link.Read<ViewBlendingModeInfo>(&info);
 
-			picture->BeginOp(B_PIC_SET_BLENDING_MODE);
+			picture->WriteSetBlendingMode(info.sourceAlpha, info.alphaFunction);
-			picture->AddInt16((int16)info.sourceAlpha);
-			picture->AddInt16((int16)info.alphaFunction);
-			picture->EndOp();
 
 			fCurrentView->CurrentState()->SetBlendingMode(info.sourceAlpha,
 				info.alphaFunction);
 			fWindow->GetDrawingEngine()->SetBlendingMode(info.sourceAlpha,
 				info.alphaFunction);
 			break;
-		}*/
+		}
+#endif
 		default:
 			return false;
 	}