Add QQuickRenderTarget overloads to adopt an existing texture array for multiview rendering

The initial idea was to just use fromRhiRenderTarget() for this, so that the clients (e.g. the OpenXR integration) need to create a QRhiTextureRenderTarget themselves and pass that in. Whereas today such code uses fromOpenGLTexture, fromVulkanImage, etc.

In order to not push functionality we can implement within Qt Quick onto the clients, add overloads for fromOpenGLTexture, fromVulkanImage, etc. that take an array size and treat it as the view count. The implicitly created depth-stencil buffer needs to be a D24S8 texture array then.