Accelerate thumnailgenerator when loading RAW files
This removes a useless call to QImage::loadFromData() that speeds up the thumbnail generation for RAW files. Its only purpose was to look up the size of the embedded preview image which is useless because
- RAWs have an embedded thumbnail which will be preferred to the embedded preview image (provided the thumb is available and big enough), and
- if the embedded thumbnail cannot be used, QImageReader::read() will decode the embedded preview image again, check its size, scale it, etc..
Also removes a redundant comparison against MIN_PREV_SIZE as this is already assured with pixelSize.
Reviewers: Gwenview, ngraham
Reviewed By: Gwenview, ngraham
Subscribers: ngraham, Gwenview
Differential Revision: https://phabricator.kde.org/D21284