Form changes are ignored until space resize

[tinchodias]

This happens both in SDL2 and MorphicWindow hosts.

Example:

canvasForm := Form extent: 50 @ 50 depth: 32.
canvasForm fillColor: Color red.
canvasElement := BlElement new
	background: canvasForm;
	yourself.
canvasElement openInNewSpace extent: 500 @ 500.

"wait and do:"
canvasForm fillColor: Color green.

The cause is AeCanvas which has a cache for Form used as backgrounds. This cache is never reseted. On space resize, a new AeCanvas is created so new cache.