This allows callers to specify the desired rendering resolution of the
image when they aren't explicitly requesting a preset image size.
It can be used for image drivers for formats which don't have a fixed
size (e.g. pdf) so that the image is rendered in the desired resolution.
This new class QgsImageCache is the equivalent of QgsSvgCache
but for raster images.
QgsImageCache stores pre-rendered resampled versions of raster
image files, allowing efficient reuse without incurring the
cost of resampling on every render.
Additionally, it offers the other benefits QgsSvgCache has,
such as thread safety, ability to transparently download remote
images, and support for base64 encoded strings.
