Multisampled FBO shows random artifacts when resizing

[hageldave]

When resizing the FBOCanvas, sometimes there are artifacts looking like a non written texture in the upper area right.
Until now I confirmed with an empty canvas where no shaders write to the FBO and the FBO is blit to the back buffer right away that the issue persists.
Another test using a shader that draws a viewport filling qhite quad could not recreate the issue, so I'm assuming it has something to do with the drawbuffer not being fully drawn to.

As a fix the FBOCanvas can draw the clear colors using a viewport filling quad at the start of the rendering sequence.

[hageldave]

Done as of 355af9e