tdf#123697 we need to create a bitmap context with alpha

Change-Id: Ib934314e56163983e70a0b04eb667e6b8527c783
Reviewed-on: https://gerrit.libreoffice.org/73362
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>

desktop/source/lib/init.cxx

@@ -2455,7 +2455,7 @@ static void doc_paintTile(LibreOfficeKitDocument* pThis,
     }
 
 #if defined(IOS)
-    CGContextRef cgc = CGBitmapContextCreate(pBuffer, nCanvasWidth, nCanvasHeight, 8, nCanvasWidth*4, CGColorSpaceCreateDeviceRGB(), kCGImageAlphaNoneSkipFirst | kCGImageByteOrder32Little);
+    CGContextRef cgc = CGBitmapContextCreate(pBuffer, nCanvasWidth, nCanvasHeight, 8, nCanvasWidth*4, CGColorSpaceCreateDeviceRGB(), kCGImageAlphaPremultipliedFirst | kCGImageByteOrder32Little);
 
     // Use the vcl.cg tag even if this code is not in vcl, to match all other SAL_INFO logging about Core Graphics, in vcl.
     SAL_INFO( "vcl.cg", "CGBitmapContextCreate(" << nCanvasWidth << "x" << nCanvasHeight << "x32) = " << cgc );