thumbnail: save all thumbnails in SRGB color space

5ee43262 forgot about the code path for directories thumbnail

This patch makes sure all image generated are in SRGB.

This gets rid of the last:

qt.gui.imageio: libpng warning: known incorrect sRGB profile
qt.gui.imageio: libpng warning: profile matches sRGB but writing iCCP instead

warnings

cc @mircomir