Consolidate testing of subpass layered rendering. Add MVKCommandEncoder _isUsingLayeredRendering member variable. Track layered rendering enabled in MVKRPSKeyClearAtt. Rename MVKRPSKeyClearAtt::enable() and ::isEnabled() functions. Add MVKRPSKeyClearAtt::reset() function. Consolidate viewport, scissor & attachment limits. MVKImage add validation for layered rendering when used as attachment.