multisampling on Wayland with OpenGL doesn't seem to work

I wonder whether the scenario below is supposed to work:

The sample code below is drawing a rounded rectangle and a shape.

When forcing vertex antialiasing with `export QSG_ANTIALIASING_METHOD=vertex`
the rounded rectangle is drawn correctly (see screenshot) and the shape is not, as expected.

When forcing msaa antialiasing with `export QSG_ANTIALIASING_METHOD=msaa`
neither is drawn with antialiasing.

Hardcoding the sample count to 4 doesn't change the outcome.

Tested backends: wayland and xcb (with QT_QPA_PLATFORM=...).

I would also assume that this doesn't work for Qt versions before 6.7, but
I only tested it with those two.

Sample code:

```

import QtQuick
import QtQuick.Shapes

Rectangle {
    id: root
    width: 500
    height: 500
    Rectangle {
        id: rectangle
        width: 300
        height: 100
        anchors.top: root.top
        radius: 50
        border

{             width: 1             color: "blue"         }

    }

    Shape {
        width: 400
        height: 300
        anchors.top: rectangle.bottom
        ShapePath {
            strokeWidth: 1
            strokeColor: "blue"
            startX: 400; startY: 0
            PathArc

{                 x: 0; y: 0; radiusX: 100; radiusY: 100             }

        }
    }
}

```

 

QSG debug info:

```
qt.rhi.general: Created OpenGL context QSurfaceFormat(version 4.6, options QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions), depthBufferSize 24, redBufferSize 8, greenBufferSize 8, blueBufferSize 8, alphaBufferSize 0, stencilBufferSize 8, samples -1, swapBehavior QSurfaceFormat::DoubleBuffer, swapInterval 1, colorSpace QColorSpace(), profile  QSurfaceFormat::CompatibilityProfile)
qt.rhi.general: OpenGL VENDOR: Intel RENDERER: Mesa Intel(R) Graphics (ADL GT2) VERSION: 4.6 (Compatibility Profile) Mesa 24.0.9-0ubuntu0.1
qt.rhi.general: Created OpenGL context QSurfaceFormat(version 4.6, options QFlags<QSurfaceFormat::FormatOption>(DeprecatedFunctions), depthBufferSize 24, redBufferSize 8, greenBufferSize 8, blueBufferSize 8, alphaBufferSize 0, stencilBufferSize 8, samples -1, swapBehavior QSurfaceFormat::DoubleBuffer, swapInterval 1, colorSpace QColorSpace(), profile  QSurfaceFormat::CompatibilityProfile)
qt.rhi.general: OpenGL VENDOR: Intel RENDERER: Mesa Intel(R) Graphics (ADL GT2) VERSION: 4.6 (Compatibility Profile) Mesa 24.0.9-0ubuntu0.1
qt.scenegraph.general: Created QRhi 0x75087c001600 for window 0x60955d6aa720
qt.scenegraph.general: MSAA sample count for the swapchain is 1. Alpha channel requested = no.
qt.scenegraph.general: rhi texture atlas dimensions: 512x512
qt.scenegraph.general: Total time spent on pipeline creation during the lifetime of the QRhi 0x75087c001600 was 4 ms