Fix initial transparency of Web content layer

[felipeerias]

This PR is a first step towards fixing the problem that the window containing Web content appears transparent for a few seconds (issue #1766).

The problem seems to be related to the fact that we are enabling alpha blending for that layer even though it already has an opaque clear colour.

However, changing that XR flag is not enough to fully fix the issue: initially the layer appears completely black, followed by the expected clear color (but too bright).

[felipeerias]

The initialisation of the browser windows seems to conflict with some of the changes at #577

Specifically, the windows are not composited for a couple seconds after they have been created, which triggers this code:

    if (mCompositionLayerColorScaleBias != XR_NULL_HANDLE) {
        vrb::Color tintColor = layer->GetTintColor();
        if (!IsComposited() && (layer->GetClearColor().Alpha() > 0.0f)) {
            tintColor = layer->GetClearColor();
            tintColor.SetRGBA(tintColor.Red(), tintColor.Green(), tintColor.Blue(), tintColor.Alpha());
        }
        mCompositionLayerColorScaleBiasStruct.colorScale = {tintColor.Red(), tintColor.Green(), tintColor.Blue(), tintColor.Alpha()};
    }

Upon creation, the window layer has these attributes:

• composited is false
• clearColor is set to our desired color, for example rgba(0.14,0.14,0.15,1.00)
• tint is set to white (rgba(1.00,1.00,1.00,1.00)) so the content is not modified

But with this code, colorScale becomes the same as the clearColor:

• colorScale is rgba(0.14,0.14,0.15,1.00)

What this means is that the clear color is being multiplied by itself, which will make it look different than expected.

[felipeerias]

Another useful change would be to allow us to set the background color for a website, similar to GeckoView.coverUntilFirstPaint.

Edit: implemented in PR #1827