qt: do not use radius in static shadow effect if image can not be rounded
Rounding is done within the shader now. Since shaders
are only supported with RHI now, there is no rounding
when RHI is not used. In that case, we can represent
the effective radius with a read-only effectiveRadius
property which would be 0 with non-RHI (software or
openvg mode) cases.
Request review @chub. Based on !6389 (merged), only the last 4 commits are relevant.