qt: fix cover art is not visible in player view with Qt >= 6.5 and software renderer
MultiEffect based DynamicShadow hides the source item because it renders the source item itself again anyway. However, it does not set itself invisible when it can not be used (software renderer).
The binding that makes the source item invisible checks if the effect is visible. Here, we can simply bind the visibility to shader type where the graphics backend needs to support RHI shader for the effect to work. This is currently OpenGL, Vulkan, Metal, DirectX; so no support for Software or OpenVG scene graph adaptations which are not targeted by Qt RHI for obvious reasons.
Request review @chub.