[Exe Thumbnailer] Workaround Qt bug in ico detection heuristic

Ico files mostly contain PNGs these days which retain their image header when embedded into an ICO file.
As a result the bits per pixel in the ICO header can be missing or incorrect.
Qt still manages to extract the underlying PNG just fine but the canRead gives up prematurely.

Differential Revision: https://phabricator.kde.org/D15819