KiteBitmapCache

Platform bitmaps that a canvas built from images, so that an image drawn many times on a page, such as a tiled background or a row of icons, converts once (#117). A canvas keeps one cache for as long as it draws.

A bitmap is kept for one image instance and one KiteImageSampling: the same image drawn at another size needs another bitmap. The cache holds at most budgetBytes of bitmaps and drops the bitmap used least recently first. A bitmap larger than the whole budget is not kept.