Summary:
If a custom folder icon is smaller than the requested preview size
`KIO::PreviewJob` returns a pixmap of only that size. E.g. `dolphin`
scales this up to its current thumbnail size which results in ugly
overlay icons for very small folder icons. If the folder icon is
scaled up before creating the overlay icons the overall preview
looks much better.

CCBUG: 315983

Test Plan:
# Set a small custom icon (e.g. 32 px) for a folder containing images
# Start dolphin with `Preview` enabled in the parent folder
# Zoom thumbnails to e.g. 128 pixels overlay
# The overlayed preview icons should not be blurry

| Before:    | After:     |
| {F5837958} | {F5837960} |

Reviewers: #frameworks, rkflx

Reviewed By: rkflx

Subscribers: ngraham, rkflx

Differential Revision: https://phabricator.kde.org/D12734