CachedImageAtSize never hits at device zoom != 100 (points compared against pixels)

[vogella]

CachedImageAtSize.refresh converts the requested draw size to pixels:

int scaledWidth = DPIUtil.pointToPixel(destWidth, DPIUtil.getDeviceZoom());

and hands it to isReusable, which compares it against the cached image's width.
That field is in points: loadImageAtSize builds the cached image with
new Image(device, imageData, getDeviceZoom()), and Image.init(ImageData, zoom)
sets this.width = round(imageDataWidth / scaleFactor).

At zoom 200 the comparison is scaledWidth/2 == scaledWidth, never true. The
single-entry size cache therefore never hits at any zoom other than 100, so every
scaled GC.drawImage misses and reloads the image.

Measured with strace -f -e trace=openat, 100 draws of one Image at a stable draw
size through the 5-arg overload, SVG:

Device zoom	Opens per draw	Total over 100 draws
100	0.02	2 (one miss, then hits)
200	2.00	200 (a miss every draw)

#3506 makes each miss cheaper but not rarer, and #3510 removes a different zoom-200
open; neither corrects the units.

Cocoa and win32 are unaffected: cocoa's cached image keeps pixels (init gets zoom
100), and win32's HandleAtSize compares the requested size against the requested size.