Pass on hasDefaultAlphaBuffer in the format with the software backend too

The expectation with setDefaultAlphaBuffer(true) is that alphaBufferSize
becomes > 0 in the window's format. Follow this with the software
backend too. (because it may matter for the backingstore)

Pick-to: 6.9 6.8
Task-number: QTBUG-136755
Change-Id: I07ff34056ea51182c2542a259084dcd746d0450e
Reviewed-by: Andy Nichols <[email protected]>

Author: alpqr

src/quick/scenegraph/adaptations/software/qsgsoftwarecontext.cpp

@@ -71,6 +71,8 @@ QSurfaceFormat QSGSoftwareContext::defaultSurfaceFormat() const
     format.setRenderableType(QSurfaceFormat::DefaultRenderableType);
     format.setMajorVersion(0);
     format.setMinorVersion(0);
+    if (QQuickWindow::hasDefaultAlphaBuffer())
+        format.setAlphaBufferSize(8);
     return format;
 }