webOS: Share images even if cache is false

Regardless Image::cache property, pixmap will be shared between images having same source. Image::cache property had two aspect of internal functionality.

1. Share image instance if source file is same.
2. Store cache for a while even if unreferenced.

So we couldn't choose one of them with existing the cache property. In webOS, sharing image instance is quite natural. So by default, the sharing is always supported regardless the cache property. And the property will only cares the 2nd functionality to store cache.

https://codereview.qt-project.org/c/qt/qtdeclarative/+/288655