Improve gallery progressive thumbnail loading speed

[JonaRichter]

Description

Gallery thumbnails load progressively but could be faster. Currently the drain timer processes 1 thumbnail per 0ms tick, and background thread does sequential cv2.imread + cvtColor + resize per image.

Possible improvements

• Process multiple thumbnails per drain tick (batch drain)
• Parallelise background imread across multiple threads (thread pool)
• Decode only the thumbnail resolution for TIFF/JPEG (libtiff/PIL pyramid read)
• Skip cvtColor on already-RGB images (e.g. PNG loaded via PIL)

Current bottleneck

For a 20-image dataset the background thread reads and resizes all images serially. On slow storage or large TIFFs this makes the last images appear noticeably later than the first.