Missing Error When Disabling Clear on Auto-MSAA Canvas

[bjornbytes]

Currently if you do Pass:setClear(false) on a pass where the canvas textures have samples == 1 but the canvas has samples > 1, the canvas is still cleared. This is pretty common to run into and it's confusing. We don't have an easy way of loading previous texture contents in this situation, so we should just error.

Workaround is to disable multisampling on the pass or render to actual multisampled textures.