Fix BitmapCanvas compositing error when drawing images.

[ferhatb]

Added endOfPaint call on recording canvas to allow bitmap canvas to dispose after reuse.
Moved all immediate canvas calls from BitmapCanvas into CanvasPool.
Reduced context updates by creating ContextHandle class.
Added regression tests for drawing on top of image in a single Picture.

Fixes flutter/flutter#44845
Fixes flutter/flutter#48032

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