OpenXR: Use a non-sRGB view for the Metal sRGB swapchain image

The Metal backend prefers the RGBA8Unorm_sRGB swapchain format like the
other backends do, but unlike them it passed the sRGB format as the view
format as well. Qt's output is already sRGB-encoded, so the GPU encoded
it a second time on write and the image came out too bright.

Pass the non-sRGB sibling as the view format, which is what the D3D,
Vulkan and OpenGL backends do by stripping the sRGB variant from the
format they hand to QQuickRenderTarget.

Pick-to: 6.12
Change-Id: I7cba3cdafecc32a3188e9ba8efeb98489619335a
Reviewed-by: Andy Nichols <andy.nichols@qt.io>