Do not depend so strongly on Form

[tinchodias]

Moreover, Form needs to be converted pixel-by-pixel to cairo_image_surface for rendering, AeCanvas "tackles" this non-cheap conversion with a LRUCache, but it's not ideal. I want to propose a change for Bloc where there is not a so strong dependency to Form. I mean this API stays available: element background: (BlImagePatternPaint form: aForm) and BlImagePatternPaint will cache the cairo surface after the next drawing. In some way, this is similar with text, where the glyphs and measurement are cached in the spans.

Related to: pharo-graphics/Alexandrie#80

Originally posted by @tinchodias in #147

[tinchodias]

Summary of related open issues about Form-to-Cairo conversion and clean up in background&paint hierarchies:

• AeCanvas misses API to directly use AeCairoSurface as background Alexandrie#80
• Form changes are ignored until space resize #660
• BlTransparentBackground can have opacity > 0 #729
• Composite background: test or delete? #728
• Delete BlTransparentPaint #721